This wasn’t in celebration of Final Fantasy VII’s anniversary or anything like that. In Japan, most fictional characters have official birthdays, and they’re all recorded on this website. Cloud’s official birthday is August 11th and so Nomura decided to mark the occasion.
The sketch that you can see above was tweeted out by the Final Fantasy VII G-Bike Twitter account, of all things, which took the opportunity to remark on his upcoming rebirth in the Final Fantasy VII remake. So, er, happy belated birthday, Cloud.