Sunday is called Sunday because it has its roots in ancient religious and cultural traditions. The name "Sunday" comes from Old English and is derived from the word "Sunnandæg," which means "day of the Sun."
In many ancient civilizations, including those of the Greeks and Romans, the Sun was associated with a prominent deity. The Sun was considered a powerful and important celestial body, and its worship was prevalent in various cultures. In Norse mythology, for example, the Sun was personified as the goddess Sól, who rode her chariot across the sky each day.
As Christianity spread across Europe, the early Christian Church adapted the existing seven-day week, which was already in use by different cultures, to fit its religious practices. Each day of the week was assigned a specific significance and associated with either celestial bodies or Germanic and Roman gods. Sunday was designated as the first day of the week, symbolizing the Christian belief in the resurrection of Jesus Christ, which took place on a Sunday according to the New Testament.
Over time, the name "Sunday" became widely accepted and used in various languages and cultures to refer to the first day of the week. While different cultures may have their own names for the days of the week, the association of Sunday with the Sun is a common theme across many languages.