DuoLingo is right. The Sabbath is on Saturday.
The Jews (who created the term) observe their holy day on Saturday (i.e. the seventh day of the week, when God supposedly rested).
If you think that Sunday is the Sabbath because you're a Christian, read the applicable passage (Matthew 28:1) again. The Bible clearly states that the first day of the week is "after the Sabbath" (i.e. the Sabbath is Saturday). That all other Christians observe the Sabbath on the wrong day is what the Seventh Day Adventists are on about.
True, but that doesn't change the fact that Judaism (the origin of the term) observes it on Saturday or that Judeo-Christian tradition treats Sunday as the first day of the week.
Beyond its ISO origins, Monday as the first day of the week stems from Slavic tradition, which didn't feature a Sabbath (well, at least not until the Christians showed up).