I made a thread here two years ago for the first Mii March, but since that thread's so old now I figured it'd be best to make a new one.
This video explains what Mii March is:
https://www.youtube.com/watch?v=6qiOCbSJqG8
During Mii March, Nintendo fans are encouraged to change their online profile pictures to their Mii characters for the month. Basically, Mii March is just a fun way for Nintendo fans to join together for a month to show their love for Nintendo, much like many Pokémon fans do in December for Pokémon Profile Picture Month. It's not some kind of promotion or anything, it's just for fun! It's not unlike NeoGAF's own Real Pic January.
If you don't know how to get an image of your Mii, here are instructions:
HOW TO GET AN IMAGE OF YOUR Mii:
On Nintendo 3DS:
1. Ensure there is an SD Card inserted into the Nintendo 3DS before continuing.
2. Select the Mii Maker icon on the HOME Menu, and tap "Open."
3. Tap the "QR Code/Image Options" button.
4. Tap "Save Mii as Image."
5. Select the desired Mii Character, press "OK," and select "Face."
On Wii U:
1. Ensure there is an SD Card inserted into the Wii U before continuing.
2. From the Wii U Menu, select "Mii Maker."
3. Tap "QR Code/Image Options."
4. Tap "Save Mii as Image."
5. Select the desired Mii Character, press "Create," and select "Face."
Once you've saved the image of your Mii to the SD Card, you can copy the image from your SD Card to your computer, or you can upload it directly from your Nintendo 3DS via the Internet Browser (uploading images saved to an SD Card isn't supported on the Wii U Internet Browser).
The easiest way to upload photos from your Nintendo 3DS is to visit https://i.nintendo.net/ from the Nintendo 3DS Internet Browser, where you can upload photos from your SD Card directly to Twitter, Facebook, and Tumblr.
If you'd like, you can even get creative and use an image of your Mii from games that utilize Mii characters, such as Tomodachi Life, Animal Crossing: New Leaf, Super Smash Bros. for Nintendo 3DS / Wii U, Nintendo Land, and more! You can use an image that you've shared to Miiverse (https://miiverse.nintendo.net/), or you can upload a screenshot from suspended software on Wii U directly to Twitter, Facebook, or Tumblr by visiting https://i.nintendo.net/ from the Wii U Internet Browser.
Alternatively, you can just visit https://miiverse.nintendo.net from your computer, sign in, click on your User Page, and either save or copy the image address of your Mii there. This works great for NeoGAF avatars, and Mii images obtained this way even have transparent backgrounds, as well!
If you're participating on Facebook, here's an event page that you can invite your friends to, if you'd like, and you can use the hashtag #MiiMarch to spread the word on Twitter and other sites.
Gaffers have participated in previous years, so hopefully the same will happen this year, too!