Seventh Day Adventists - Can you please explain this Sunday worship at a SDA church to me?

The Seventh Day Adventist Church of Tonga worships the Sabbath on Sundays... why? Isn't Sunday worship the "mark of the beast". Confused?? I would appreciate your understanding of this, thank you in advance :-) God Bless
Update: Thanks Mary,,, so am I. It is going to be very important to me soon and am seeking clarification.
Update 2: Thank you for the link Magdiel :-)
Update 3: My understanding is that the SDA church in Tonga has received heavy criticism for this practice from the SDA leaders and members, as it enforces the Sunday worship, yet it continues. All churches there worship on the same day... what does this mean? That Sunday law already exists around us??? I am not sure on the... show more My understanding is that the SDA church in Tonga has received heavy criticism for this practice from the SDA leaders and members, as it enforces the Sunday worship, yet it continues.

All churches there worship on the same day... what does this mean? That Sunday law already exists around us??? I am not sure on the date line aspect, which has recently changed. But it just seems to go against what the SDA church believes, as per my understanding :-)
Update 4: Thank you so much Novacom...... very, very, very helpful to me.
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