As usual, getting to Heathrow over the next few weeks is not necessarily going to be straightforward, and those used to public transport services running on Christmas Day may be surprised at how LHR is quite restricted in this area.

The official guide is now out here, the link is the best place to go in case of any updates, however I've appended the PDF to this post just in case.
https://www.heathrow.com/content/dam...port-guide.pdf

In summary, focusing on how to get to and from Central London to the airport:

Christmas Eve - mostly normal services, however rail and tube services will terminate earlier than normal. Basically OK to 20:00 hrs, thereafter it's best to check. The Heathrow Express (HEX) should operate (despite the PDF being unclear on this).

Christmas Day - National Express are running a 30 minute service from Victoria Coach station to T2/T3, then T5 then T4. Tickets available online and you can use this travel between terminals. However there is a train service between terminals landside, and the airside buses will be operating. No HEX, Elizabeth Line (EL) or Piccadilly line services.

Boxing Day / St. Stephen's Day - Piccadilly Line is working from about 07:20, no HEX or EL, other than the inter terminal services.

27 to 29 December - No HEX, but restricted EL service and inter terminal service. Piccadilly Line is on a Saturday service.

30 December - HEX returns to service from the start of the day, EL and Piccadilly running normally.

Thereafter for New Year's Eve, New Year's Day and onwards, normal service is scheduled on HEX, EL and Piccadilly.

This has happened before, but a number of Piccadilly line trains have been damaged by leaf fall on the Middlesex sectors, and while they seem to be returning to service, plus this period has less pressure on passenger numbers, nevertheless it's best to allow an extra 20 minutes on top of normal timings.

Hat tip: Nick_USA