This is just something that popped into my head while I was staring at the wall the other day. I guess they're not all technically "Christmas" songs but they tend to get called that anyway so. . .
A few things:
1. The answers are all found the same way.
2. 1,3,and 5, have the spaces marked. 2, 4, and 6 do not.
3. "Key" may not be the right term. You do not absolutely need the key to figure out the titles but it might make things a little easier.
1. oharna caob obarhmgbe hsinbmg kfsink Key: 58, 11, 474, 110, 59
2. plipalbhmgmgmnbchfcaocscmg Key: 57585594
3. vb vfko mnpsc h albararmn lioarfkcaalhk Key: 3, 15, 22, 1, 362, 5867
4. carpkcamncaobksipvalhsi Key: 7253878
5. beblina caob ohmgmgk Key: 90, 11, 117
6. popmgmnsifnoca Key: 15710
Good luck!