Resist the apparently nearly uncontrollable urge to ask "is this available"
Start by telling me when you will come. If you have a change of plans, you can let me know. No big deal.
I'm not available Friday after 4pm. Saturday night after 8 might work.
I'm probably available all day, but I'm not going to know until that day if I'm doing something. If you want to come a day other than the day you message on, I'll probably just say 4pm to 7pm.