Tangled - The Lanterns

Rapunzel has lived in that tower for pretty much her entire life until she was kidnapped by Mother Gothel as a baby. On her 18th birthday she wants to leave the tower because she believes she is old enough and she want to find out what the floating lanterns are about and if they are for her. If she has never spoken to anybody else other than Mother Gothel she wouldn't have known about birthdays unless Mother Gothel told her, so why did she even tell her what birthdays were? Mother Gothel is quite smart so she should have realised that Rapunzel would eventually figure it out because I'm pretty sure most people would get curious as to why everyone in town had a party every year on your birthday. I know Rapunzel reads a lot of books and she may have found out from them but I would have thought that Mother Gothel would have read the books beforehand to check that there was nothing in it that would make her realise that she is the Princess. If Mother Gothel hadn't told Rapunzel about birthdays she wouldn't have left the tower and Mother Gothel would probably still be alive.