Tenant just text me will be late on rent for the first time

[David A.]

I have had this couple always be on time for the past 6 months but I just received a text. I am not sure what to think about it because I really want to be strict about late rent and in the lease agreement says that late rent will be charged a fee per day. Here is the text
"Hey David i just want to give a heads up I'm going to be a few days late this month I will have rent on Tuesday sorry".
What should I do should?? thanks