Let the festivities begin!

Less than a week before they're expected to tie the knot, Ashlee Simpson and Evan Ross got into the celebratory mood on Tuesday with a dinner party for Ross' 26th birthday.

Joined by Evan's famous mama Diana Ross and 13 others, the crew dined at STK Downtown, with everyone in "great spirits," a witness said.

There was "lots of wedding chatter" and "tons of PDA" from the engaged couple, the spy added.

Earlier in the day, Simpson, 29, and her beau were spotted at Bowlmor Chelsea Piers.