We don't really know when Jesus was born.
Urantia book says he was born the 21th of August of Year 7 BC but we don't know if this date refers to gregorian calendar or julian calendar.
Anyway, the nativity of Jesus is celebrated in christmas the 25th of December. In general, before Jesus birth, the holiest day of the year was christmas. This is why we celebrate Jesus birthday the 25th of December.