If a triggered ability says 'at the beginning of your upkeep,' does it trigger if you skip your upkeep?

A triggered ability that says 'at the beginning of your upkeep' uses the beginning of your upkeep step as its trigger event. For this trigger to fire, that step must actually occur — specifically, it triggers at the very start of the upkeep step (CR 502.2). If the upkeep step is skipped, that moment never happens, and the trigger condition is never met.

CR 500.10 addresses skipping steps and phases: when a step or phase is skipped, it is treated as though it simply does not occur during that turn. No game events that would happen during that step are processed, including triggered ability checks (CR 603.2).

Skipping upkeep can happen due to effects like Stasis or other cards that instruct a player to skip their upkeep. In all such cases, any 'at the beginning of your upkeep' triggers — such as cumulative upkeep costs, Howling Mine variants, or enchantments with upkeep triggers — do not trigger for that turn.

Concrete example: You control Mesmeric Orb and Stasis. Stasis causes you to skip your upkeep. Because your upkeep step never occurs this turn, Mesmeric Orb's 'at the beginning of your upkeep' ability does not trigger, and you do not mill any cards that turn.

Note: If an effect instead says you skip 'the untap step' or another specific step, your upkeep still happens normally and its triggers still fire. The skip must specifically apply to the upkeep step for this rule to apply.