Is it rare for identical twins to start there periods on the same day?!

that is pretty normal actually since your the same age and at the same points of development so for your cycles to be really close to each other is not a huge surprise

and actually... its been shown that a group of women who work in a close group like an office can have their periods change in timing to match the other women in the group
this happens as a biological thing... to compete for a mate, when women sense or know that another woman is at that time , their bodies will adjust so they can send the signal to the potential mate that they are also ready

so.. no.. not rare at all