Sounds more like a question you should ask your league. The season officially ends when the last out is recorded. Be it on Sunday or Monday. If your league follows those rules as far as free agent pickups, then no one will be able to claim a player after the last out of the regular season is recorded. None of us know how your league is set up though, so best if you ask your commish. If he doesn't know, then the league needs to get it figured out quick.
Yes doctor, I am sick.
Sick of those who are spineless.
Sick of those who feel self-entitled.
Sick of those who are hypocrites.
Yes doctor, an army is forming.
Yes doctor, there will be a war.
Yes doctor, there will be blood.....