Steadfast Lutherans has a wonderful series of posts by Pr. Joseph Abrahamson called "Redeeming Holy Days from Pagan Lies," refuting the oft-made claim that Christian holidays are all of pagan origins. Here is a list. Issues, Etc. has also interviewed Pr. Abrahamson on these topics.
Christmas and Sol Invictus
Solstice 1 and 2 and 3
Ash Wednesday and Lent
Easter 1 and 2
Nativity of St. John 1 and 2 and 3