How far ahead should I rent a table or in can only be done on open days ?

This time of year I would go for at least 2 weeks ahead

Office is only open Thursday- Sunday and this time a year you need to rent about 3 weeks ahead

You're going to want to do it seven days out and they are in the office every day so you can call whenever

I would say call them and ask. I personally have never looked into renting a table there.