cracker

Well, despite the name, it only stops when the initiator decides to, and informally does not last a day.

If you say it's Opposite Day after the initiator stops Opposite Day, then you are the new initiator and can stop anytime.
However, if it's said after the initiator says it's Opposite Day, it's still going to be Opposite Day since you're not the initiator who said it was Opposite Day in the first place, and Opposite Day can continue until the initiator says otherwise.

No

This is what scientists call the opposite day paradox

then its same night?

IDK