Rookie Mormon

Why do Mormons worship on Sunday and not Saturday?

It's actually this:

  • Mormons in Muslim Nations: Hold church on Fridays
  • Mormons in Israel: Hold church on Saturdays
  • Mormons in Other Nations (including USA): Hold church on Sundays

You can read more about this topic at the FAIR LDS wiki page: Sabbath changed to Sunday. Here is a quote from that page:

Latter-day Saints do not base their worship practices on an analysis of early Christian history, or on the comments of scholars in Biblical commentaries, though these sources can confirm Church teachings. Rather, the Saints follow the guidance of a living prophet. However, it seems clear that the Latter-day Saint practice of observing the day of rest and worship on Sunday—like most of the Christian world—is consistent with the earliest Christian practice of which we have record.

The article also explains that Mormon congregations in Israel worship on Saturdays, and LDS congregations in Muslim nations worship on Fridays.

Clearly, the Lord is far more concerned that His people worship Him regularly, and that they set aside a day to dedicate to him. He does not wish us to contend about a matter as trivial as the day dedicated to his worship.(See: 3 Nephi 11:29-30Colossians 2:16.)

...

The modern Church, guided by prophets and apostles, does not seek contention with others over the "proper" day of worship; rather, they invite all to worship and come unto Christ. This tends to be done on the day which accords best with the practices and patterns of the culture in which they find themselves.