Lindsay Lohan -- Rehab Is Gonna Ruin My Birthday!!!!

Don't you just hate it when court-ordered rehab totally ruins your birthday plans? Lindsay Lohan does ... and is telling friends she's super bummed she'll be turning 27 while on lockdown.

There's no getting around it ... Lindsay has been ordered to check-in to a rehab facility for 90 days within the next month and half, so her July 2 birthday WILL fall in the treatment zone ... and she's pissed.

To be clear ... not about the whole rehab thing ... just about missing her birthday (seriously).

We're told LiLo had big plans -- which involved throwing a party in NYC -- but now it's all gone to the crapper after she copped a plea deal with prosecutors and took rehab over jail

In case you forgot, LiLo has already been to rehab five times since 2007 -- including one stint during her 21st bday as well -- but none have stuck. Sources close to LiLo tell us, they're hoping this next trip will finally sober her up.