I didn't fill the meter on MLK day not knowing it was required, and received a fine. Any way to contest it?

It appears that MLK, Columbus Day and Veterans Day are the only three holidays where meters are enforced. The meter was actually part of a pilot pay-by-cellphone program in the area, so my ticket says "No dp placard expired meter did not pay by cell". Can I push the angle that this is discriminatory to those who cannot afford a cellphone, or that I am offended MLK is relegated to a second-rate holiday? Or should I just be honest and say I had no clue but have learnt my lesson?

Parking offenses, like most traffic offenses, are "strict liability". This means you do not need to have knowledge you are doing something wrong.

Because you chose to park there, you agree to abide by the rules, even if those rules require a cell phone.

That does not mean you can't try your angle... contesting the ticket will probably not cost you any more.

Pay the darn ticket / Lesson learned.....

It seems to me that you should have known about the program before you parked there for free. You can always contest it and hope that the judge you get is in a good mood, but it's probably best to just pay it and learn your lesson.

You probably didn't have to go to work/school on MLK day so you could spend your entire day honoring his memory. You DID spend the day reflecting on his greatness, DIDN'T YOU???
Unless you got a parking fine in front of a library, church, homeless shelter or a bordello (hey, MLK had vast interest/experience in EVERYTHING), suck up & pay the fine.

California Governemnt Code, sections 11120 and 54950. Just to help you a little(if you in fact live in California). Aslo check with the secretary of states office to validate and make sure that the issuing law enforcement agency is in fact a legally registered business entity within said State. If not consult the advice of an ATTORNEY. This is just to help you. kinda get you started on a very eye opening experience. This is only advice. Enter at your own capacity.