In this 13th century painting (click on to view larger), the crescent moon at Mary's foot is lifted from ancient Egyptian symbology, where it was an attribute of the goddess Isis, the Queen of Heaven, writes David Fontana in The Secret Language of Symbols(1).
He notes that holidays such as Christmas and Easter coincide with pagan festivals and that many of the objects typically associated with Christmas, "such as the yule log, mistletoe, and the Christmas tree itself, were all borrowed from pre-Christian traditions."
1. http://www.amazon.com/exec/obidos/ASIN/0811804623/ctoc