Jeff Russell

That's a tricky question, and the fact that the day ended up staying "Saturn's Day" suggests that the ancient Germanic folks didn't have a fully satisfactory answer. Some folks have suggested one or another of the Jotnar as an equivalent, based on mythic parallels - say, Ymir or Mimir. One complicating factor is that Saturn, as worshipped by the Romans, may or may not have had that much to do with Kronos in the Greek myths - some of the parallels the Romans accepted seem less obvious to us than others, and less justified on modern comparative mythological grounds, and even where the parallel seems pretty good, the details of the cultus to that God often differed quite a bit between the Greeks and the Romans - for example, Mars and Ares seem to have had rather different worship associated with them.

Another approach I've seen a lot of heathens take is to throw up their hands, say "there's no equivalent, so I can worship Whomever I want on Saturdays" - Galina Krasskova describes her take on this in Devotional Polytheism (she uses Saturday for Loki). My own practice veers this way - I try to do something to honor the God of the day on each day of the week, and for Saturday, I've picked Idun, as She is personally important to me, and if you squint, has some similar associations to Saturn, at least as the Romans worshipped Him.

Personally, I've mostly let go of attempts to arrive at a "pure" "untainted" Germanic practice - syncretism and incorporation of various kinds were very much the norm among historic polytheists, though it didn't end up as the kind of bland, uncareful "eclecticism" you sometimes see among modern polytheists. I haven't gone so far as to offer worship to Saturn directly, but it would be entirely historically normal to bring in worship of one God from another culture if you had a personal relationship with Him.

I'd welcome anyone else's thoughts on this, as it's something I'm not wholly satisfied with my own answer on.

Cheers,
Jeff