Sunday is the first day of the week 

What happened on the first day of the week? 

"And God said, Let there be light: and there was light." - Genesis 1:3 KJV 

When God manifested in flesh to die for the sins of the world, He brought mercy and came as the light. 

"Then spake Jesus again unto them, saying, I am the light of the world: he that followeth me shall not walk in darkness, but shall have the light of life." - John 8:12 KJV 

When we are born again of water and of the Spirit by repentance, water baptism in Jesus name, and receiving the Holy Ghost, we become the children of light. (John 1:12-13, John 3:5, Acts 2:38) 

"While ye have light, believe in the light, that ye may be the children of light..." - John 12:36 KJV 

It was on the first day of the week that Jesus was resurrected and when He first appeared to His disciples. 

"Now when Jesus was risen early the first day of the week, he appeared first to Mary Magdalene, out of whom he had cast seven devils." - Mark 16:9 KJV 

"Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you." - John 20:19 KJV 

The disciples assembled together on the first day of the week. 

"And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight." - Acts 20:7 KJV 

"Upon the first day of the week let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come." - 1 Corinthians 16:2 KJV 

I was glad when they said unto me, Let us go into the house of the Lord!