I stopped to question why the NDOW prepositional subcommittee chose on. Yes, the appropriate preposition doesn't announce itself--you can quickly get rid of amongst and betwixt, but why on as opposed to, say, of or for?
On implies something more philosophical, more inquisitive than the functional of or for. On a day of we do. On a day on we meditate, we consider, we discuss. And there's something contemplative about the project; as the site says:
That's a project we can all support. Write on.