Actually, originally the Celts held Samhain "on or around" the 31st of October, and it's only when it was absorbed and linked to All Saints Day that it was given a specific day on the calendar.
That said, if there was a blizzard that snowed in my family for Christmas, and we couldn't get together for gift giving until the 5th of January, my Christmas would be January 5th that year. Because holidays aren't about the calendar, they're about community.