2018-12-07 : If the target creature becomes an illegal target for Seize the Day, the spell doesn’t resolve. There isn’t an additional combat phase or main phase.

2018-12-07 : If you somehow cast Seize the Day during an opponent’s main phase, that opponent attacks during the resulting combat phase, since it’s still that player’s turn. If you somehow cast it not during a main phase at all, all that happens is you untap the target creature.

2018-12-07 : Seize the Day can target a creature that’s already untapped.