Sunday (noun)

1. The day of the week before Monday and following Saturday, observed as the Christian Sabbath and as a day of rest and worship.
2. A particular Sunday on which something happens or is marked for special observance.

Origin:

Old english sunnandæg, from sunnan, from the genitive singular of sunne sun + dæg day; akin to old high german sunnuntag sunday, latin dies solis, greek hemera heliou.

Examples:

1. I always go to church on Sunday.
2. We usually have a big family dinner on Sundays.
3. Today is Sunday, the last day of the weekend.
4. He had a day off on Sunday.
5. The store is closed on Sundays.