Can I pay you Friday?

Hi everyone I wanted to see if this was happening to anyone else! I am having clients text me during my busiest season 1 or 2 hours before their 2 hour appointment to see if she can pay me another day. I get so frustrated when they do this because if I say no, I now have 3 hour gap in the middle of my day. What would you do in this situation? It seems to be happening more and more lately.