As every year there are several holiday calendars.
The ones I follow this year are:

• 24 Ways – Web design and development articles and tutorials for advent
• MDN Holidays calendar by Chris Heilmann
• 24 Pull Requests – Giving back little gifts of code for Christmas
• 25 Days of Making – faberdashery

There are two more in German:

• Webkrauts Adventskalender 2013
• Hangout Adventskalender