503.1
First, any abilities that trigger at the beginning of the upkeep step and any abilities that triggered during the turn's untap step go on the stack. (See rule 603, "Handling Triggered Abilities.")
Historical Rules · Effective 1 November 2013
Read Section 503 continuously, or open any rule on its dedicated page.
Chapter 5 · Turn Structure
Section 503
3 rules
First, any abilities that trigger at the beginning of the upkeep step and any abilities that triggered during the turn's untap step go on the stack. (See rule 603, "Handling Triggered Abilities.")
Second, the active player gets priority. Players may cast spells and activate abilities.
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.