1. Of, relating to, or associated with Sunday
2 [from the practice of wearing one's best clothes on Sunday to attend church] Best
3. Amateur

1. (Homonym: sundae).
2. First day of the week; observed as a day of rest and worship by most Christians; Also called: Lord's Day, Sun.
First day of the week; in Christianity, Sunday is set aside for worship in commemoration of Jesus' resurrection, and in predominantly Christian societies banks, offices, and many shops are generally closed. It replaced the Jewish Sabbath, or day of rest, observed on Saturday.
“Blue laws,” which restricted commercial activities, entertainment, and the purveying of alcoholic beverages on Sunday, have been repealed in many parts of the us since the 1950s.

To spend Sunday; SYN. Sunday.