put the flags out
or put out the flags
BRITISH1. If you put the flags out or put out the flags, you celebrate something special that has happened. Even now, they must be putting the flags out in holiday resorts like Bognor and Blackpool. Birthdays and christenings, or just a spell of good weather, are all the excuse you need to put out the flags and celebrate summer in the garden.
2. People say Put the flags out! to show that they are surprised but pleased that something has happened. The builders have finished? Put the flags out!
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012