my labrador is in season and i dont know the best time for her to do it

The best time is when she's been successfully hip-scored and eye-tested (as has the stud) and you've researched her pedigree for compatability with the sire.

Also she'll be about 3 years old.

And when you've read the Book of the Bitch from cover to cover........

And when you've had someone knowledgable see her (preferably in the show ring) to assess whether she is of sufficient quality to breed from.