Celebration is a condition-checking ability word (not a true keyword), so it has no inherent rules of its own. It triggers a bonus when "two or more lands entered the battlefield under your control this turn." Each card that uses Celebration spells out its own effect, but they all share that same check.
The most common misunderstanding: it counts lands, not permanents in general, and it requires the lands to have entered this turn under your control—not lands you simply have in play. It also doesn't matter how they entered; playing a land for the turn plus any extra land drop, a fetch effect, or a "put a land into play" spell all count.
In Commander, it's easy to enable with ramp staples, extra-land-drop effects, and fetch lands. Just sequence your lands carefully so both hit the same turn you want the bonus.