On the first day Jesus rose from the dead (Mk. 16:9).
On the first day Jesus first appeared to his disciples (Mk. 16:9).
On the first day Jesus met with the disciples at different places and repeatedly (Mk. 16:9-11; Mt. 28:8-10; Lk. 24:34; Mk. 16:12-13; Jn. 20:19-23).
On the first day Jesus blessed the disciples (Jn. 20:19).
On the first day Jesus imparted to the disciples the gift of the Holy Spirit (Jn. 20:22).
On the first day Jesus commissioned the disciples to preach the gospel to all the world (Jn. 20:21; with Mk. 16:9-15).
On the first day Jesus ascended to Heaven, was seated at the right hand of the Father and was made Head of all (Jn. 20:17; Ep. 1:20).
On the first day many of the dead saints arose from the grave (Mt. 27:52-53).
The first day became the day of joy and rejoicing to the disciples (Jn. 20:20; Lk. 24:41).
On the first day the gospel of the risen Christ was first preached (Lk. 24:34).
On the first day Jesus explained the Scriptures to the disciples (Lk. 24:27,45).
On the first day the purchase of our redemption was completed (Ro. 4:25).
On the first day the Holy Spirit descended (Ac. 2:1). Pentecost was on the 50th day after the sabbath following the wave offering (Le. 23:15,16). Thus Pentecost was always on a Sunday.
The Christians met to worship on the first day (Ac. 20:6,7; 1 Co. 16:2). Sunday is “the Lord’s Day” (Re. 1:10)