So how often do schools cancel on contracted subs at the last moment?

Today is my 2nd day subbing for Kelly Education. Yesterday went fine, all day sub for middle school. Day 2, they cancel me at 7:30 in the morning (my class is at 1). It's frustrating because I am a single dad so I build my schedule around this a bit. No call or anything, no explanation, just an inconspicuous email.

I had read about this happening but I am still surprised it happened on literally my 2nd day. How frequently does this happen? I am wondering if I should get out of this line of work ASAP or not.

Thank you.