There's a "missionary" (?) trying to evangelize this cult at my school (an American state school).
They generally come at you starting with this argument:
The Sabbath is Saturday. Jesus and the Apostles worshiped on the Sabbath (in Jewish context, but never mind that). Therefore, the Sabbath remains the "correct" day for worship services. If your church gathers on a Sunday, then it must not be the true Church.
They have a "Bible study" that they try to recruit you to.
Embed related for a summary of their polemic.