By the early-to-mid 4th century , the Western Christian Church had placed Christmas on December 25 , a date later adopted in the East , although some churches celebrate on the December 25 of the older Julian calendar , which corresponds to January in the modern-day Gregorian calendar .
In such regions , Christmas is celebrated on December 25 in the Julian Calendar , but because of the difference between the calendars it is the 7th of January in the modern Gregorian Calendar .