12/26 is official from the IRS side of things:

MeF Production Shutdown Schedule:

• The shutdown for “Send Submissions” will begin at 11:59 a.m. Eastern Time on Friday, December 26, 2025.
• Transmitters may continue to use all other service requests, except “Send Submissions,” until 11:59 p.m. Eastern Time on Friday, December 26, 2025.
• States may continue to use all service requests necessary to retrieve and process state submissions, in accordance with the shutdown activities noted above. MeF recommends that states adjust their processing schedules to ensure submissions are retrieved in time to validate returns and issue acknowledgments.
• Any acknowledgments not retrieved by 11:59 p.m. Eastern Time on December 26, 2025, will not be accessible until MeF reopens for production in January 2026.