Find an emotion that needs social approval in order to be easily expressed.
Hook it into something you sell or do.
Discover other organizations that would benefit from the holiday as much as you would.
Voila! Mother's Day/Valentine's Day/Festivus/New Year's. It doesn't have to be a national one, of course, just one for your tribe.
All the great religious holidays started as secular or pagan holidays first, because they filled an essential social need. Spring is here! It's dark out!
And if your project/product/cause isn't worthy of a holiday? Time to find a new one.