Your/somebody’s luck is in
Definition 1 of 1
Pro Tip 1/3Positive Vibes
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 2/3Not Literal
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 3/3Casual Use
Mostly used in casual, informal conversations rather than formal settings. 
Looks like your luck is in with the weather for your garden party!