Hi All

i work as a self-employed cleaner and am 9 weeks into working with a fab agency who are giving me loads of hours. It's all working out really well.

The thing is, i have Therapy every week on a Monday.

I need to tell the Agency boss lady that i cannot work Mondays, but don't want to tell her the reason why.
The thing is, she is a very scary lady! I worry that she will decide to 'let me go' if i say i can't work on Mondays.

I have already had 1 day off due to sickness, and had to swap 1 day for another when i had car trouble. but i REALLY need these Mondays off, every week, without fail.
My therapist cannot do any other day.

How do i say what i want in an assertive way?