There is no mention of worship here....only eating a meal and they came together on the FIRST of the week. It does not say first day!
Well, an understanding of Greek constructs would help one know the word "day" is implied in the phrase.
It's like saying, "I'll meet you at Billy's BBQ Hut at three."
One does not need to say "three o'clock" for most folks to know what one means.