Most contemporary writing is set in the eternal present. It means that when thereader reads the book, it is set in modern day. It also means the contemporary writer does not have to worry about when things happened.
Christmas to them is moveable feast, set on whichever day of the week, it happens to fall in the year that the reader is reading it.
Not so with the historical writer as often a specific year is chosen. Thus there are readers who can quickly look up what was happening in 1846 -- the year my current wip is set in. Christmas was on a Friday for any who are interested. Also means I have to take into account dates that things might be closed, in order to give the wip an authentic feel.
If I was writing a contemporary, I would be able to be more vague. The Monday after the first Sunday in Advent for example, but not in this one. As it is anchored to a specific time, events happen on specific dates. One has to work with it, rather than against it.
And I know the vast majority of people will not be bother whether on Christmas1846 fell on aFriday or not. But some will and I am. Thus it adds to my enjoyment of the writing to try to give it as an authentic flavour as possible.