In short
The Sabbath
After six days of creation God rested on the seventh day and made the Sabbath for all people as a memorial of creation. The fourth commandment asks us to keep holy the seventh day, from Friday sunset to Saturday sunset, as a day of rest, worship and service.
Read it in the Bible
Genesis 2:1-3; Exodus 20:8-11; Luke 4:16; Isaiah 58:13, 14; Hebrews 4:1-11; Leviticus 23:32
This is a short summary. The church's full statements are at adventist.org, and our pastors are glad to study any belief with you.