Ashton Kutcher white derby cap

You're not supposed to wear white after Labor Day ... unless it's Yom Kippur.

Ashton Kutcher and pal Guy Oseary donned head-to-toe white -- which symbolizes purity in Jewish culture -- while leaving the Beverly Hills Kabbalah Center on Saturday.

Ashton's girlfriend Mila Kunis didn't attend the service, but perhaps ex Demi Moore was inside?