laugardagr is the Old Norse word. Swedish developed from Old Norse (as did Norwegian and Danish). The Swedish lördag is derived from laugardagr.

The “laug” portion comes from Old High German “louga”, from PIE root leue- "to wash."

The “dagr” portion comes from the PIE root agh- "a day" with an initial “d” added as part of Old High German. OHG is the ancestor of Old Norse.

This is from Etymonline - a great resource for language information https://www.etymonline.com/search?q=laug

I think mongol is right to be careful. There wasn't one coherent old Norse, it had east-west variation even then for example.

But it should give the broad strokes, just like talking about PIE is certainly a broad stroke.