If you are considered single for tax purpose when the year ends, you can file as single. Even if you get your final divorce decree at 11:59 pm on December 31, you can file as single.
And, even if...
To the bank. The practice of keeping them aside (and awarding them to people for landing on, say, Free Parking) is a house rule that is actually severely detrimental to the game. A lot of people hate...