Sun·day 
(sŭndē, -dā′)
(sŭndē, -dā′)n. Abbr. Sun. or S
The day of the week that comes after Saturday and before Monday. It is considered the first day of the week in Judeo-Christian tradition. Sunday is observed as the Sabbath in most branches of Christianity.
[Middle English sonnedai, sondai, from Old English sunnandæg; see sāwel- in the Appendix of Indo-European roots.]