5.  Turn Structure

503.  Upkeep Step

503.1  The upkeep step has no turn-based actions. Once it begins, the active player gets priority. (See rule 116, "Timing and Priority.")

503.1a  Any abilities that triggered during the untap step and any abilities that triggered at the beginning of the upkeep are put onto the stack before the active player gets priority; the order in which they triggered doesn’t matter. (See rule 603, "Handling Triggered Abilities.")

503.2  If a spell states that it may be cast only "after [a player’s] upkeep step," and the turn has multiple upkeep steps, that spell may be cast any time after the first upkeep step ends.