It's ON! Justin Bieber and Hailey Baldwin -- who've already been married for a year, as we all know -- jumped on a private jet Saturday headed for South Carolina where the couple will wed on Monday, in their "official" religious ceremony after doing a quickie courthouse wedding in New York last year.

Justin spent his last hours in LA visiting his therapist, which seemed to put him in a bit of a bad mood. As he left the Beverly Hills medical building he was annoyed with photographers and called them "ants" and "mosquitos."

Meanwhile Hailey was lunching with her stylist, grabbing some new looks to take on her trip to the east coast for one the most important days of her life! Her style has GOT to be on-point!

Then yesterday afternoon, they soon-to-be-nuptialed Mr. and Mrs. Bieber hopped on a private jet, with the help of their entourage loading up plenty of luggage and wedding gear -- and took off for the south east.

We can't wait to see photos from the big day!