holy day; the Sabbath
Primarily a religious term referring to a day set apart for worship, especially the Sabbath in Judeo-Christian contexts.
キリスト教徒にとって、日曜日は聖日です。
For Christians, Sunday is a holy day.
ユダヤ教で
In Judaism, Saturday is considered the Sabbath.
Compound of 聖 (holy, sacred) and 日 (day). The reading せいじつ follows standard on'yomi.