I'm booked in a regular economy seat for a flight tomorrow (Sunday) evening. Normally I fly on EMB 145 flights with no Business/First, so I normally check in exactly 24 hrs before flight time to increase my chance of an economy plus seat. If there is a United First seat available when will it clear - 24 hours before or on the day of departure. If the latter, should I wait to check in until the flight day?

Thank all!