What error do you get exactly?
To help diagnosing the problem, try this:
MySql="Update Calendar Set booked = true, UID= " & Session("id") & ", treatment ='" & request.form("treatment") & "', Comments='" & request.form("comments") & "' Where id=" & id
That outputs your SQL statement to the browser. Then copy and paste the statement in the query designer of your database, to see where it breaks.
If that doesn't help, can you post the outcome of the Response.Write statement?