Here are a few resources regarding Christmas (and other non-Muslim festivals):
1) From the Permanent Committee - may Allaah preserve it:
a) Participating in Kafir's feasts
b) Celebrating with Christians in their feasts
c) Congratulating Kafirs in their feasts
d) Ruling on attending Christians' festivities (Arabic version)
e) More (Arabic version)
2) "Responding to 'Merry Christmas'" by Sh. 'Uthaymeen - rahimahullaah (summerized & translated by brother Rasheed Barbee - hafidhahullaah; see below)
3) "A Word on Non-Muslim Holidays" mainly from Sh. 'Abdullaah Bassam's - rahimahullaah - work; translated by brother Jamil Finch - may Allaah reward him.
4) Audios on Christmas (for purchase via SalafiAudio):
a) Reality of Christmas (Sh. Muhammad al-Madkhalee - hafidhahullaah)
b) Should Muslims Celebrate Christmas? (Bro. Abu Khadeejah - hafidhahullaah)
c) The Realities of Christmas (Bro. Abu Hakeem Bilaal Davis - hafidhahullaah)
In Arabic:
تحذير المسلمين من الاحتفال بأعياد الكافرين - "Warning the Muslims from Celebrating the Festivals of the Disbelievers" - includes speech of 'Ulemaa' of the Past including Ibn Taymiyyah & Ibn Qayyim - rahimahumallaah.