Awesome Advent

User Experience

• http://uxmas.com An advent calendar for UX folk.
• http://2016.leanuxmas.com The top articles from 2016's Agile & Lean UX News

Art

• https://www.itsashapechristmas.co.uk/ Shape Christmas is a digital advent calendar that counts down to christmas. Revealing bespoke artwork each day themed around 4 shapes.

Frontend development

• http://web.advent.today Collection of 25 writings by web designers and developers, on things they are excited about.
• https://24ways.org is the advent calendar for web geeks. For twenty-four days each December we publish a daily dose of web design and development goodness to bring you all a little Christmas cheer.
• http://grochtdreis.de/advent-2016/ Codepen experiments from Jens Grochtdreis
• http://calendar.perfplanet.com/2016/ Performance Advent Calendar
• http://christmasexperiments.com/ Advent calendar mostly about WebGL

Programming

• http://adventofcode.com Advent of Code is a series of small programming puzzles for a variety of skill levels. They are self-contained and are just as appropriate for an expert who wants to stay sharp as they are for a beginner who is just learning to code. Each puzzle calls upon different skills and has two parts that build on a theme.
• https://react.christmas/ A collection of articles on React

Programming languages

• https://24daysindecember.net PHP advent calendar
• http://perladvent.org/2016/ Perl advent calendar
• https://perl6advent.wordpress.com Another Perl advent calendar
• https://blog.gopheracademy.com/series/advent-2016/ Go Advent Calendar

Sysadmin

• http://sysadvent.blogspot.nl With the goals of of sharing, openness, and mentoring, we aim to provide great articles about systems administration topics written by fellow sysadmins.

Content Strategy

• https://gathercontent.com/advent-calendar Advent calendar where content strategy experts share advice, talk about their hot content topics and reveal their focus for 2017.