Set up a reoccurring event on your calendar, create a git page of some sort and suggest developers to donate to on whatever day you choose.

Yes, it's today. Donate now.

Having like an annual day where floss folks talk about projects they're excited about and show them some love would be really cool actually

For you to remember, perhaps not needed. As Reannlegge mentioned, if a banking or payment service you use allow setting recurring payments, that's a good option. Also if the developer or developers are on platforms that allow recurring payments, like Patreon, it's also interesting.

For reminding others, when talking about such projects, include a reminder to support every now and then, though avoid spamming or speaking only very sporadically.

I think there February the 14th I love free software day by the FSF