Your/somebody’s luck is in

Pro Tip 1/3

The phrase gives a cheerful feeling, suggesting things are going well.

You've found a twenty-dollar bill on the ground - your luck is in!

Pro Tip 2/3

Doesn't always mean literal luck; can imply things are simply going favorably.

You arrived just as they restocked the sale items; your luck is in!

Pro Tip 3/3

Mostly used in casual, informal conversations rather than formal settings.

Looks like your luck is in with the weather for your garden party!