501. Beginning Phase
In short
The beginning phase: just a collective term for the untap, upkeep and draw steps (rules 502–504). "At the beginning of your turn" means, in rules terms, "at the beginning of your upkeep".
Informal summary — the official English text below is authoritative.
501.1.The beginning phase consists of three steps, in this order: untap, upkeep, and draw.
Source: Wizards of the Coast, Magic: The Gathering Comprehensive Rules. Unofficial presentation under the Fan Content Policy. Rules effective as of Sep 25, 2026