Hi Anne,
As Chris has already said Noël and Pâques don't have an article in front of them but the Saints days do ,
e.g. C'est la St Jean aujourdh'ui or aujourd'hui c'est la Sainte Honorine , ou la Sainte Isabelle, ou la Saint Martin...
It is ( so and so ) Saint' s day today ...
You also use the article with other religious celebrations , e.g. La Toussaint, La Pentecôte, l'Ascention . ( All Saints' day, Pentecost/ Whitsun , Ascention day )
So Noël and Pâques are pretty much the exceptions here,
Hope this helps!