Sunday is a noun that is defined as the seventh day of the week and is traditionally a day of rest. It is often kept as a day of religious observances and, depending on each country or culture, may include activities such as attending church, shopping, leisure activities, or spending time with family.
We always have a family breakfast on Sunday.
I usually spend Sunday mornings in church.
Sunday is always capitalized.
Sunday is also employed as an adjective in most cases. Its role and function is to describe a thing, activity, or person associated with Sunday.
I prefer Sunday shopping.
You can enjoy some Sunday evening walks.