"Tuesday" which sounds like "twos-day" is day number 2 in the week whether you one-index and start your week on Monday or zero-index and start your week on Sunday.

It's from Tyr, the Norse god. Originally closer to Tyr's day, hence the odd spelling relative to the pronunciation.

Likewise we have (W)Odin's Day, Thor's Day, Frigg's Day, and Saturn-Day. Of course there's also the Sun's day and the Moon's day.