How can I change language on calendar?

Hello, i am from Poland but the site I`m creating is in English. On Reservation formula there is a calendar - everything is in English accept a small box with in which the specific date is being chosen - there is still Polish language (Listopad = November, dzisiaj = Today). I can`t change it. Please give me some instructions how to do that.

Thank you

Best Regards

Ania

It looks like you changed the date field:

Let us know if you need more help.