I always forget how many important dates there are in December... Luckily, G. Thomas Fitzpatrick doesn't! Here's a list of the dates from his post...
Important feasts celebrated during December include:
1st St. Edmund Campion
3rd St. Francis Xavier
4th St. Barbara
6th St. Nicholas
7th St. Ambrose
8th The Immaculate Conception
9th Ven Fulton Sheen
12th Our Lady of Guadalupe
13th St. Lucy
16th St. Adelaide
21st St. Thomas the Apostle and St. Peter Canisius
22nd St. Frances Xavier Cabrini
23rd St. John Cantius
24th Christmas Eve and Adam & Eve
25th Christmas
26th St. Stephen
27th St. John the Apostle
28th Holy Innocents
29th St. Thomas a Becket
And as previously mentioned, this year's Advent's Embertide is the 16th, 18th and 19th of December.