I was wondering if anybody knows when it will likely be possible to buy LNER tickets from London to Sheffield for dates falling around Christmas (21 December to 27 December 2019). I was also wondering if there would be reduced service, and what prices will be like.
According to National Rail Enquiries, there are no train services on Christmas Day, 25th December, and limited services on Boxing Day, 26th December.
There are two key web pages you should use to check your specific journeys, the Journey planner and Future engineering work. Unfortunately, a lot of the more disruptive engineering work is scheduled around Christmas. When looking at the 27th, be aware that sometimes work scheduled for the 26th does not finish on time.
Traveller indicated in a comment that tickets from 16th December onwards are estimated to go on sale on 1st November. You can set an alert when tickets become available at Tickets for future travel dates.
Just to fill out the options mentioned in comments.
There are two routes available: one served by East Midlands Railway from London St Pancras and one from London King's Cross served by London North Eastern Railway involving a change at Doncaster or Retford or possibly Grantham (I did not do an exhaustive check) onto a Northern service to Sheffield. The direct service is approximately 10% less journey time and about 10% cheaper depending greatly on whether you can get a cheap advance ticket or not.
At holiday periods when there is often engineering work it is worth checking for work on both the main London to Scotland lines as if one has major works the other will be very crowded with people re-routing to Scotland.