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