Is it true Christians don't have to keep the whole law (Galatians 5:3)?

What does whole law mean in Galatians 5:3? What does the "manner (custom) of Moses" mean in Acts 15:1? Is it true Jesus came to show us we don't have to keep the Sabbath and Holy Days? Did God change His law through Jesus Christ? Do we just keep 9 of the 10 commandments because Jesus died for us? Since Jesus' death and resurrection, is the Sabbath just "in our hearts" and we just rest in our minds?