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Programming is an activity where you need to continuously learn to stay productive. Programming languages, libraries, tools, operating systems - they all change. This means that last year’s solutions that you find on the Internet might not be applicable this year. Thus we need new content (be it blog posts, documentation, books or other forms) showing the current best way to achieve a given goal. One solution is StackOverflow with its voting system (where out-of-date solutions get voted down and working solutions get voted up) and others are blogs. The problem with blogs is that people rarely have time to blog frequently and/or don’t have expertise and a diverse set of domains, thus readership interest can waver. There are tries to encourage people to blog regularly (such as the Perl Iron Man Blogging Challenge), however, it is easier to sustain a blog with multiple authors (also called collaborative blogging).
A programming Advent calendar is such a collaborative blog centering around one language / framework / library or subsection of the IT field. Just as you open one door of an Advent calendar every day between the 1st and 24th of December to find a surprise, these blogs publish one article each day for the first 24 days of December about their respective topics surprising, informing and delighting their readers.
Some of the programming Advent calendars are:
Java Advent Calendar http://www.javaadvent.com/
I’m partial to this, having started it 4 years ago 
. It contains articles about all kinds of technologies related to the Java Virtual Machine, not just Java the language. For example, we had articles about two of the oldest languages running on the JVM (besides Java): NetRexx and Kawa.
The Perl 6 Advent Calendar https://perl6advent.wordpress.com/
Perl 6 is a new programming language to be released this Christmas, so if you’re interested in Perl 5, this is a good site to follow
Perladvent http://www.perladvent.org/
If you’re not ready to make the jump to Perl 6 yet (since it’s a completely new language, not just a superset of Perl 5), here you can find all kind of interesting details about it.
24ways https://24ways.org/
“24 ways 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.”
Sysadvent https://sysadvent.blogspot.com
Interesting articles for system administrators and anyone curious about the field of IT.
UXMas http://uxmas.com/
“An Advent calendar for UX folk.”
Perl Dancer Advent Calendar http://advent.perldancer.org
“The PerlDancer Advent Calendar is a community-driven project that aims to showcase the Dancer Perl web framework.”
24 Pull Requests http://24pullrequests.com/
This isn’t a collaborative blog, but rather an effort to encourage people to contribute to free / open-source software on GitHub
Qiita Advent calendars http://qiita.com/advent-calendar
There are a lot of topics covered here (and I mean a lot - in 2014 they had 214 different Advent calendars with different topics). Unfortunately, you need to be able to read Japanese to enjoy it.
These are the ones which are currently active (there were others which unfortunately are no longer available).
You can subscribe to any of these using your favorite RSS reader (for example Liferea, RSSOwl, feedly, NewsBlur and so on) to ensure that you never miss a post. Some of them (like JavaAdvent) also offer other means of subscription (like Twitter, Facebook, Google+ or even email).
Finally, all of these sites welcome (and in fact depend on) user contribution. So go ahead and subscribe to them and also consider writing an article or two yourself! As they say: the best way to understand something is to try to explain it to others!