You would think so, but actually, it's Celtic in origin. I was reading up on this, and it comes from the Proto-Celtic for "this day": https://en.wiktionary.org/wiki/heddiw
I also read somewhere this morning that Welsh was one of a few languages that had the initial 's' change into an 'h' though I tried to find the reference and now I can't. Does anyone have an idea if that's actually true, or just something my brain cooked up? :-D
I don't know about Welsh specifically, but I DO know of that same sound change appearing somewhere else. Proto-Indo-European initial *s would become Ancient Greek /h/, hence the cognate to English "six" being ἕξ /heks/.
I don't have my Gateway to Sindarin on me at the moment, but (for what it's worth) I believe in the internal history of Tolkien's Welsh-inspired Sindarin langauge the OPPOSITE happened, hence the soft mutation of initial Sindarin s- being h-.
Neither. Welsh "heddiw" and Latin "hodie" (whence comes Esperanto "hodiaŭ") are cognates but not borrowings from each other. The Welsh word comes from Proto-Celtic and eventually Proto-Indo-European (PIE), which is the common ancestor of the Latin word. They evolved separately.