According to the us constitution, who decides when the electoral college casts its vote for president?

Sorry, my first answer was deleted. I can re-word it in a way where it won't be though.

The Congress (Legislative branch) decides when it is time for the electoral college to vote

(U.S. Constitution, Article 2)

hope this helps.

The people who decide when the electoral college casts its vote for president is congress because they have been bestowed the power to do that by the US Constitution.