George Clooney and Rande Gerber Get Birthday Surprise from Amal

George Clooney and Rande Gerber kept a tradition going of celebrating their birthdays, thanks to Amal -- who surprised the hell outta both of 'em over the weekend.

Sources close to the group tell TMZ ... Amal invited Rande and Cindy Crawford to fly to the UK to surprise George, and join them for an intimate Saturday night dinner. We're told it worked ... George was dumbfounded when he saw his bff.

George and Rande -- whose birthdays fall within about a week of each other -- usually celebrate together, but since George and Amal moved to London ... it seemed like that might not happen.

Rande's appearance wasn't the only surprise. We're told Amal brought out a custom Casamigos cake -- featuring mini George and Rande figurines ... which actually kinda does 'em justice.