The Christian group The Seventh-day Adventist Church is called so is because of its belief in the second coming of Christ (i.e. the Advent). The other reason explains the phrase "Seventh Day". Unlike most other Christian denominations, believers observe the Sabbath (i.e. the day of rest in Semitic religions) on the Saturday of a week, not the Sunday. This is akin to how Jews mark the Sabbath.
This year, in the end of December, a group of Seventh-day Adventists in the little island country of Samoa were faced with a dilemma around this observance, and eventually most parishes decided to observe it on a Sunday from now on. Why?
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Samoa is switching to the west of the International Date Line (the imaginary boundary between calendar days in the Pacific) so as to share the same time zone as its major economic partners such as Australia and New Zealand. This meant they moved from Thursday (29th) to Saturday (30th), losing Friday. The church members begin the Sabbath from Friday evening, which won't be possible this time. To preserve a 7-day cycle, they have decided to switch to a Sunday.
Religion was never easy. More here.