MEMX Goes 23x5: The Real Deadline Is the SIPs' December 6 Launch

MEMX Joins the 23x5 Race, but the SIPs Set the Clock

MEMX overnight trading rules are already effective, but the 9:00 p.m. Overnight Trading Session cannot open until the SIPs can carry overnight data, currently targeted for December 6, 2026, and MEMX files a follow-on rule change confirming that readiness. The rule change, SR-MEMX-2026-28, adds a 9:00 p.m. to 4:00 a.m. ET session and moves MEMX to 23-hour, five-day trading. The SEC published the notice as Release No. 34-106310 in the Federal Register on September 14, 2026. The notice sets October 5, 2026 as the comment deadline.

An effective rule is not a live rule. The SEC's June 26, 2026 order approving the CTA/CQ Plan amendment (Release No. 34-105779) states that certain exchange Participants "have not started trading during the Exchange Extended Hours" because their rules bar them from doing so until the Equity Data Plans are ready. MEMX cannot open the overnight session until the securities information processors, the SIPs, can disseminate overnight quotes and trades. The SIPs' planned production launch is December 6, 2026. That is the date your trading desk, supervisory procedures, and client disclosures need to be ready for.

Inside SR-MEMX-2026-28: Four Sessions Built on a Copied Template

MEMX filed SR-MEMX-2026-28 on August 28, 2026, under Section 19(b)(1) of the Exchange Act and Rule 19b-4. The filing produces this daily schedule:

• Overnight Trading Session: 9:00 p.m. to 4:00 a.m. ET, starting on a night before a business day.
• Pre-Market: 4:00 to 9:30 a.m. ET.
• Regular Trading Hours: 9:30 a.m. to 4:00 p.m. ET.
• Post-Market: 4:00 to 8:00 p.m. ET.

The design is not original, and MEMX does not claim it is. The notice says the proposal is based substantially on Cboe EDGX's 23x5 proposal. According to the notice, the SEC approved that proposal in Release No. 34-105587 on May 29, 2026. The notice states that "the basis for the proposal is materially similar" to EDGX and that the filing "does not raise any novel or unique issues not previously considered by the Commission."

That copying is the signal. A September 29, 2026 WilmerHale alert counts MEMX among five exchanges with conditional approval for 23x5 trading, contingent on SIP readiness: 24X, Cboe EDGX, MEMX, Nasdaq and NYSE Arca. This particular filing took effect on filing under Rule 19b-4(f)(6). Overnight equity trading is becoming a standard exchange product, not a niche ATS offering.

Effective on Filing Is Not the Same as Open for Business

Operating teams often treat these two events as one. They should not.

Effectiveness is a legal status. MEMX's rules took effect when the exchange filed them under Rule 19b-4(f)(6), which also makes a non-controversial change operative 30 days after filing unless the Commission shortens that period. The comment period runs alongside effectiveness rather than in front of it. Under Exchange Act Section 19(b)(3)(C), the Commission may summarily suspend the rule change at any time within 60 days of filing.

Operation is a market-infrastructure event. Under the proposed rule text, MEMX will not start the Overnight Trading Session until the Equity Data Plans can collect, consolidate, process and disseminate quotes and trades during overnight hours. Before it opens that session, MEMX must also file a separate Section 19(b) proposed rule change confirming that it and the Equity Data Plans are ready. If MEMX does not file its related Overnight Trading Session Proposed Rule Change within 18 months of effectiveness, it must promptly file to remove the overnight rules. That is a built-in sunset.

The SIP side already has its approvals. On June 26, 2026, the SEC approved the CTA/CQ Plan amendment (Release No. 34-105779) and the UTP Plan amendment (Release No. 34-105780). The UTP Plan amendment extends the Processor's hours of operation to run from 9:00 p.m. ET Sunday to 8:00 p.m. ET Friday, with a one-hour pause at 8:00 p.m. ET Monday through Thursday. The CTA/CQ order records the expectation that "the implementation of the amendment will occur on December 6, 2026."

December 6 is still a target. It remains contingent on SIP readiness.

The Broker-Dealer Compliance Problem Overnight Trading Creates

The commercial pitch for overnight trading is investor access. The compliance reality is that every firm routing equity orders will soon operate in a market with consolidated overnight data, multiple lit exchange venues, and a regulator that expects procedures to reflect both. SEC staff analysis, cited by WilmerHale, puts current overnight volume at roughly 0.9% of NMS share volume, growing 359% year over year. A firm that routes overnight orders without overnight procedures will have no documented answer when an examiner samples a 2:00 a.m. fill.

Where the exposure sits

• Best execution. FINRA Rule 5310 requires reasonable diligence to find the best market for a customer order. Once the SIPs publish overnight quotes, a consolidated overnight NBBO gives examiners a benchmark for 2:00 a.m. fills that did not exist when overnight trading lived only on ATSs.
• Supervision coverage. Procedures written around a 4:00 a.m. to 8:00 p.m. day do not cover a desk taking orders at 9:00 p.m. on a Sunday.
• Market-wide halts. MEMX filed separately on circuit-breaker resumption. Under SR-MEMX-2026-29, after a Level 3 halt, trading does not resume until 4:00 a.m. ET or later on the following trading day; your order-handling systems need to reflect that. MEMX retained the 4:00 a.m. resumption even though it would otherwise begin trading at 9:00 p.m. under 23x5.
• Customer disclosure. The filing amends MEMX's extended-hours customer disclosure rule in the proposed rule text. As amended, the rule provides that "No Member may accept an order from a customer for execution in the Overnight Trading Session, Pre-Market or Post-Market Session without disclosing to such customer that extended hours trading involves material trading risks, including the possibility of lower liquidity, high volatility, changing prices, unlinked markets, an exaggerated effect from news announcements, wider spreads and any other relevant risk." The amended text adds overnight-specific risks, including that regulatory protections "may be more limited or different than those available during Regular Trading Hours."

Registered investment advisers face the same problem through a fiduciary lens. An adviser whose broker routes client orders overnight should be able to explain why that venue and that timing served the client.

What Your Leadership Team Should Decide Before December 6

The decisions are operational, and each one has an owner.

Decisions for broker-dealers

1. Decide whether you will offer overnight trading at all. Staying out is a legitimate choice, but it should be documented. It should not happen by default.
2. Assign supervisory coverage by name. Identify who reviews exceptions, halts, and complaints during the 9:00 p.m. to 4:00 a.m. ET window.
3. Rebuild your best execution review. Add overnight venues and SIP data to your regular and rigorous review before the first live session, not after.
4. Test halt and resumption logic. Run scenarios against the circuit-breaker procedures MEMX filed in SR-MEMX-2026-29.

Decisions for advisers and fund managers

• Ask your executing brokers in writing how they will route and supervise client orders overnight.
• Update trading and allocation policies so overnight fills are captured in your best execution review.
• Revisit client disclosures if any strategy will trade outside regular hours.

The board-level question is simple: if an examiner asks for overnight procedures on December 7, what does the firm hand over?

Key Takeaways

• Effective does not mean live. SR-MEMX-2026-28 took effect on filing, but the overnight session cannot open until the SIPs can disseminate overnight data and MEMX files its follow-on readiness proposal.
• December 6, 2026 is the date to plan around. It is the date the CTA/CQ order records as the expected implementation of the SIPs' overnight amendment, and it remains contingent on SIP readiness.
• Overnight trading is becoming standard. WilmerHale counts five exchanges with conditional approval for 23x5 trading, and MEMX based its session design substantially on Cboe EDGX's approved 23x5 rules.
• Best execution reviews need a new time zone. A consolidated overnight NBBO gives examiners a benchmark that did not exist when overnight trading lived only on ATSs.
• The rules carry a sunset. MEMX must file to remove the overnight rules if its follow-on filing does not come within 18 months.