Lindsay Lohan -- BIG Birthday Plans ... With Her Lawyer

Lindsay Lohan is celebrating her birthday today with the one person in her life she can count on ... her lawyer Shawn Holley.

Sources close to LiLo tell TMZ ... Cliffside -- where she's currently in rehab -- denied the actress' request to have friends visit on her day of birth (i.e. today). Visiting days are only on Sundays ... rules are rules.

We're told LiLo's plan B was asking if she could leave the facility to get dinner with friends (with the DA's permission of course). Again she was shot down.

But all hope was not lost ... certain people can visit patients outside of visiting hours, like lawyers. And we're told Shawn is headed to the Malibu rehab joint to hang out with LiLo while she sulks over b-day #27.

As TMZ previously reported, LiLo was already pissed she'd be on lockdown for her birthday ... she wanted to have a huge party in NYC.

We'd say better luck next year, but y'know ...