Psalm 118:24

The people singing Psalm 118 at a temple festival meant something specific by “this day”: one particular day on the calendar, tied to an actual deliverance their community remembered and had gathered to celebrate. A few verses earlier the psalm speaks of “the stone which the builders rejected” becoming the cornerstone (verse 22), a line all four Gospels later apply to Jesus.

Centuries of use have stretched “this day” well past that one festival, until it gets sung or quoted over any ordinary Tuesday morning. What makes that stretch work is the reasoning behind the original line: the God being praised for one specific day is the same God present on every other one, so the rejoicing carries over even though the original occasion doesn't.