I would like to create a reminder on Google calendar every month at the 15, but if the 15 in that month is Friday/Saturday, I want it automatically to set it to the closet Sunday/Thursday. Is it possible?
AFAIK this is not possible in Google Calendar.
A possible workaround might be to create the reminder for the 14th, which would catch all Friday-situations (if I understand correctly, you would want a notification on Thursday if the 15th is a Friday, and one on Sunday if the 15th falls on a Saturday). This would always give you a notification on-time, even if too early in most months.