January is named after the God Janus, who faced both forward and backward. The New Year starts with the 1st of January, and many cultures and traditions had traditions of cancelling old debts, taking on new habits, and making resolutions.
The usual ones are about eating better, losing weight, flossing, etc. which fundamentally boil down to improving one's health. Gyms are jam-packed for the first two weeks of January, and February is a GREAT time to get good deals on weights and treadmills on Craigslist.
Veganuary is a registered charity which challenges people to vow not to consume anything animal-based, to live vegan, for at least January. Whether people want improved health, to combat climate change, are dealing with a diagnosis of heart disease or just want to reduce animal suffering - the charity and its website is there to inspire that lifestyle change.
It's doing so in partnership with many vegan businesses and concerned organizations - offering supermarket, eating out, recipe info. The recipes aren't just "eat a plate of greens and be sad and boring" but actual gourmet chef recipes that use no animal products.
2017 was a banner year for veganism. In addition to it becoming a minor fad, it became really big business. Mega-mergers and purchases have turned what were once niche products for a group Anthony Bourdain compared to Hezbollah into a genuine bona fide business. Gardein sells in just about every grocery, and vegan products routinely sell out. Even Hellman's and Ben and Jerry's make vegan options, and they're fantastic.
The charity and accompanying site have made it dead simple to join in.