I was just speaking to a Seventh Day Adventist about why we celebrate the Eucharist on Sunday. A longer answer could be given, but I thought I’d just post here the quotation I just shared with him from St. Justin Martyr:
We hold our assembly on Sunday because it is the first day [of the week], on which God brought forth the world from darkness and matter. On the same day, Jesus Christ our Saviour rose from the dead. For He was crucified on the day before [Saturday]; and on the day of the Sun [Sunday[ he appeared to His apostles and disciples and taught them these things, which we have submitted to you for your consideration. - St. Justin Martyr, First Apology (c. AD 150)
If you would like to read a larger extract from St. Justin where he describes Christian worship in Rome during the Second Century, please see the Patristics Section of this website.