First and Next Step?

[JoblessandStressed]

I recently was terminated for not showing up on my scheduled day off. This was on a Friday. Wednesday, my boss asked if I could come in on Friday to work. I stated I would be unavailable since I had made plans, being it was my scheduled day off (all of this being stated while two other employees were present). My boss proceeded to schedule me in anyways. Before leaving my shift for the day, I restated a second time that I would be unavailable Friday.

Having reread the company handbook and policies, the following is the only portion I found that even remotely relates to this situation:
Employees unable to report for work due to illness or emergency must report to their Supervisor 1 hour before their normal starting time the day of their absence.

Thursday I sent a text message to my boss, stating for a third time, that I would be unavailable for the shift on Friday. Later in the evening my boss called me, asking if my text message meant I was quitting, to which I stated "No, I was just letting her know that I was still unavailable for any shift on Friday." She replied that she was happy to hear I wasn't quitting and okay on the not available for the Friday shift.

Friday, when I went in to pick up my paycheck, she called me to her office and proceeded to tell me she had been informed by her boss that I was to be "let go."

I know of another employee that did not show up for a "On Call" shift and no actions were taken against her yet, I am terminated for not coming in on my scheduled day off.

Aside from the obvious discrimination, what is the next step I should take?