Looking on the internet deeply has found these results:

I am available on any day is the most popular phrase on the web.

I am available at any day 10 results on the web	I am available on any day 112 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	I am available on any day that is convenient for Ms. Whoever (note: or, "I am available on date, date or date"). I am eager to come in, and look ... 15/hr. October, I have appointments available Sundays ... wedding cake season, May Now whilst you may not be superstitious, and I am not – touch wood, I am available on any day of the week including the lucky early days to perform the ... Tuesdays. From November – March I am available on any day for appointments. I am taking a break from college until at least September so I am available on any day, even on short notice. I can do any household task su... $10

I am available at any day

10 results on the web

I am available on any day

112 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

I am available on any day that is convenient for Ms. Whoever (note: or, "I am available on date, date or date"). I am eager to come in, and look ...
15/hr.
October, I have appointments available Sundays
... wedding cake season, May
Now whilst you may not be superstitious, and I am not – touch wood, I am available on any day of the week including the lucky early days to perform the ...
Tuesdays. From November – March I am available on any day for appointments.
I am taking a break from college until at least September so I am available on any day, even on short notice. I can do any household task su... $10