GNU is a free/libre/open source operating system. It may be used with a variety of kernels. GNU started in 1984. The combination of GNU with Linux is called GNU/Linux. It is perfect to be distributed on a Software Freedom Day.
There are tons of distributions. Here is only a small list
For more distributions take a look at Distrowatch !
* For more Free Software Foundation certified libre distros, take a look at GNU/Linux-Libre FSF Freed Software !