From what I know in school, Sunday is the start of the week. I think it is just established as part of the weekend since regularly or normally we do not have work during Sunday. On th 7th day, in the bible, God rested from creating the world. On the 7th day, which is Sunday, everyone should take a rest and also visits church to hear the Gospel of the Lord. Even when I am growing old, and starting working, everyone told me that it is the start of the week. It is regard only as weekend because we do not work. I should assume the first day of the weekend, the first day of every week and so on. Weekend is the most calming part of the week and Sunday is the blessed ones for Catholics and other religions. Sunday is the least favorite of mine because I always think that this is the start of the stressful day at work.