What actually triggers a rebalance, a fixed date or a drift threshold?

Both work, and the point of either is to stop you deciding case by case, which is where emotion creeps in. A calendar approach means you check on a fixed schedule, say once a year, and nudge things back toward your target mix. A threshold approach means you only act when an asset drifts past a set band, for example when equity moves more than a chosen number of percentage points from its target. Many people combine them: look on a schedule, but only actually trade if something has drifted enough to matter. The goal is to do less, not more, rebalancing too often just racks up costs and taxable events. Since selling to rebalance can trigger capital gains, and those rules and rates change, factor in the tax cost and confirm current treatment with a CA before you sell. Pick one rule you'll actually follow and let it be boring.

A curated summary to orient you, not advice. The resources below are the real value.