Can I cast an instant after blockers are declared?

You can cast instants and spells with flash before blockers are declared. You can cast instant spells any time you have priority, including after attackers are declared and before blockers are declared (C.R. 116.1a, 508.2, 116.3d). See also this thread.

Can you cast an instant during combat?

Instants are spells that can be cast at any time, even during your opponent’s turn or during combat. Like sorceries, instants have a one-time effect, and then you put them into your graveyard.

Does declaring attackers go on the stack?

After a legal attack is declared, the active player gets priority. If anything triggered on declaring attackers, it goes on the stack here.

Can I cast instants in response to my opponent declaring?

When my opponent declares blockers, I want to be able to kill a blocking creature (with something like Hero’s Downfall) while still doing combat damage to my opponent. Is this possible? To answer the question in your title, yes, each player receives priority and can cast instants after blockers are declared.

Can you cast holy day after you declare blockers?

Short answer: Yes, he can cast those instants after you decide not to block. Yes, you can cast ‘Holy Day’ after he casts all those instants to prevent the damage. Players have a chance to cast instants and activate abilities at the end of each of those steps, including after he declares attackers and you declare blockers.

When to cast instant to prevent a creature from blocking?

The last time you can cast an instant to prevent a creature from blocking is in the Assign Attackers step, after attackers have been declared. If you have moved to the Declare Blockers step, it’s too late, since the very first thing that happens is the defender declares blockers. Nobody has priority until after that happens.

Can you cast instant spells after blockers are declared?

Yes. The beginning of the blockers phase must have blockers being declared. After that, during the rest of the phase, instants may be cast. The next phase is damage, and damage occurs at the beginning of that phase. Yes. This is how ninjitsu (the ability from kamigawa) works. Without this rule Magic would be a somewhat lesser game IMO.