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 4 February 2011
Section 503: Upkeep Step
Read Section 503 continuously, or open any rule on its dedicated page.
Chapter 5 · Turn Structure
Section 503
503. Upkeep Step
2 rules
503.2
Second, the active player gets priority. Players may cast spells and activate abilities.