The Bible never claims a date of December 25 (he and his followers primarily used the Hebrew calendar, which as a lunisolar calendar does not have a day-to-day correspondence to the fully solar Roman calendar nor its modern-day descendant Gregorian calendar); contextual clues may imply a date in or around September. The fixed date is attributed to Pope Julius the First because in the year 350 AD he declared the twenty-fifth of December the official date of celebration. The date may have initially been chosen to correspond with either the day exactly nine months after Christians believe Jesus to have been conceived, the date of the Roman winter solstice, or one of various ancient winter festivals.