Beginning phase

The beginning phase is the first phase in a turn. It consists of three steps, in order:

• Untap step
• Upkeep step
• Draw step

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Beginning Phase

Part of the turn. This phase is the first phase of the turn. See rule 501, “Beginning Phase.”

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 501. Beginning Phase
  • 501.1. The beginning phase consists of three steps, in this order: untap, upkeep, and draw.

Untap step

The untap step is the first step of the beginning phase. The following events happen during the untap step, in order:

• All permanents with phasing controlled by the active player phase out, and all phased-out permanents that were controlled by the active player simultaneously phase in.
• If the day and night cycle has started, the number of spells cast in the previous turn is checked, and if the appropriate conditions are met, day turns to night or night turns to day, including the transformation of Daybound and Nightbound permanents.
• The active player determines which permanents controlled by that player untap, then untaps all those permanents simultaneously. (The player will untap all permanents they control unless a card effect prevents this.)

No player receives priority during this step so spells or abilities cannot be played. Any state-based effects or triggered abilities that happen during this step are delayed until the upkeep step.

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Untap Step

Part of the turn. This step is the first step of the beginning phase. See rule 502, “Untap Step.”

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 502. Untap Step
  • 502.1. First, all phased-in permanents with phasing that the active player controls phase out, and all phased-out permanents that the active player controlled when they phased out phase in. This all happens simultaneously. This turn-based action doesn’t use the stack. See rule 702.26, “Phasing.”
  • 502.2. Second, if it’s day and the previous turn’s active player didn’t cast any spells during that turn, it becomes night. If it’s night and the previous turn’s active player cast two or more spells during that turn, it becomes day. If it’s neither day nor night, this check doesn’t happen and it remains neither. This turn-based action doesn’t use the stack. See rule 727, “Day and Night.”
    • 502.2a Multiplayer games using the shared team turns option use a modified rule. If it’s day and no player on the previous turn’s active team cast a spell during that turn, it becomes night. If it’s night and any player on the previous turn’s active team cast two or more spells during the previous turn, it becomes day. If it’s neither day nor night, this check doesn’t happen and it remains neither. This turn-based action doesn’t use the stack.
  • 502.3. Third, the active player determines which permanents they control will untap. Then they untap them all simultaneously. This turn-based action doesn’t use the stack. Normally, all of a player’s permanents untap, but effects can keep one or more of a player’s permanents from untapping.
  • 502.4. No player receives priority during the untap step, so no spells can be cast or resolve and no abilities can be activated or resolve. Any ability that triggers during this step will be held until the next time a player would receive priority, which is usually during the upkeep step. (See rule 503, “Upkeep Step.”)

Upkeep step

The upkeep step is the second step of the beginning phase. At the beginning of the upkeep step, any abilities that trigger either during the untap step or at the beginning of upkeep go on the stack. Then the active player gains priority the first time during their turn.

Upkeep costs were formerly a specific type of cost paid for permanents in play "during your upkeep", but are now triggered abilities prompting for payments which activate at the beginning of the upkeep step. It can be paid to gain something (Farmstead), or to prevent a negative consequence (Force of Nature), especially to avoid the sacrifice of the permanent (Nicol Bolas). This last use is what the step is named after - paying for the "upkeep" of the permanent to keep it alive - although this kind of upkeep cost is rarely used as a game mechanic anymore. Cumulative upkeep is a keyword ability variation of the mechanic.

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Upkeep Step

Part of the turn. This step is the second step of the beginning phase. See rule 503, “Upkeep Step.”

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 503. Upkeep Step
  • 503.1. The upkeep step has no turn-based actions. Once it begins, the active player gets priority. (See rule 117, “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.

Draw step

The draw step is the third step of the beginning phase. The following events occur during this phase, in order:

• The active player draws a card from their library.
• Any abilities that trigger at the beginning of the draw step go on the stack.
• The active player gains priority.

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Draw Step

Part of the turn. This step is the third and final step of the beginning phase. See rule 504, “Draw Step.”

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 504. Draw Step
  • 504.1. First, the active player draws a card. This turn-based action doesn’t use the stack.
  • 504.2. Second, the active player gets priority. (See rule 117, “Timing and Priority.”)