In the sentence, "Three students were mentioned, namely John, Sarah and Sylvia.", do you understand why 'namely' is used?
There are 'Three students' (possibly from a whole class or school of students) but they are anonymous. The 'namely' tells us that the writer is now going to identify them individually, and then gives us their names, John, Sarah and Sylvia.
"We decided to meet each other again,namely on Sunday"
In this sentence, no 'day' has been mentioned that could then be identified specifically as Sunday.
Look at this sentence:
After much debate, they decided that they would meet two days each week, namely, on Sundays and Wednesdays.
Here, 'two days' has been mentioned, and then these are specified, 'named',
as being Sunday and Wednesday.