Heading for The Hills? Don't forget your party hat â€" that was the message for guests at Lauren Conrad's 22nd birthday bash this week.

NOTE: Unlike Justin Timberlake, who actually turned 27 today (woo!), Lauren Conrad's birthday isn't until tomorrow (February 1).

But hey, why not get a head start, right?

Right. LC celebrated with her (predominantly female) posse at Hollywood hot spot Katsuya. But it wasn't all play for the reality TV star.

She stars on The Hills, after all, so crew members arrived almost an hour before Lauren Conrad's 9:45 p.m. appearance at the venue to smooth out technical issues and set up their shots â€" standard operating procedure.

When the guests arrived â€" including Lauren's old friend and new roommate Lauren Bosworth (a.k.a. "Lo," above) and Whitney Port â€" they were wearing party hats, and kept them while enjoying edamame, spicy tuna and birthday cake.

"The group toasted Conrad's birthday during their meal and couldn't have looked happier together," a witness reports.

As for the birthday girl? Lauren Conrad sipped a watermelon-cucumber mojito and "was just chatting with all of her friends and [smiling]."

After cake was served, the party dispersed and The Hills gals worked some set-ups with the crew for the show's new season. We can't wait!