I'm pretty sure some of the observances I find really only exist on the internet. I mean, National French Toast Day? Sure, I like French toast, but should it have its own day? I guess, if you're a toast-head and feel strongly enough to march on the Bastille for your right to soak bread in milk and eggs, we'll today is your day.
What's potentially the first ever recipe for French toast actually appeared in the Roman collection of recipes. Apicius. Searching You can find a 2009 translation of the work on Project Gutenberg and within its digital pages you'll find a recipe for Aliter Dulcia which loosely translates to "otherwise sweet" (at least to the best I can determine). If you're interested in cooking a 5th century version of French Toast, here's how:
Break (slice) fine white bread, crust removed, into rather large pieces which soak in milk (and beaten eggs). Fry in oil. Cover with honey and serve.
Like most early recipes, this one is short on methods and measurements. There's no micromanaging the amount of time you soak the bread, no instructions on how thick to slice the bread, nothing saying you should only use artisan oil crushed from the pits of olives from a mysterious village in Southern Umbria. We've been a culture of perfect, of Martha Stewart judgmentalism, where nothing's worth doing if it's not nearly flipping impossible and worthy of Instragram bragging rights. Anyway, if you want a more modern recipe, check out this one from the French Cook Book for American Families, by Xavier Raskin, published in 1922: