I shall answer with another question: if you do not have a significant other, do you celebrate/observe Valentine's Day?The answer, would be a yes, but...
1) You certainly might take advantage of the various discount chocolates
2) As a kid, you may have handed out various valentines
3) You might hand out cards/candy/small gifts to family and friends to show you are thinking of them.
4) Be annoyed at the focus on couples during this time frame.
5) Because of 4, rebrand Valentine's Day as Single Awareness Day.
Celebrating Christmas as a non-believer is much the same, except add the jerks that constantly talk about "the reason for the season" and "war on Christmas" to the point I've been reprimanded for using X-mas because it "took the Christ out of Christmas"...never mind that Xmas was historically used and invented by Christians for Christians...And no-one seems to mind that we don't observe the "mass" part of Christmas.
Whew! End of rant! So, in a nutshell, it's like Valentine's day for singles but with waaay more annoying interactions with people.