dayofweek

Day of the week is a designation for each day within a seven-day cycle used in most civil calendars. The common sequence is Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday. In many regions the week starts on Sunday, while international standards such as ISO 8601 place Monday as the first day of the week and often represent days numerically from 1 (Monday) to 7 (Sunday) or 0–6 in other conventions.

Etymology and naming conventions vary by language. In English, the Sunday and Monday names reflect the Sun and the Moon. The other days primarily derive from Germanic or Norse deities and figures: Tuesday from Tyr, Wednesday from Woden (Odin), Thursday from Thor, and Friday from Frigg/Freya. Saturday broadly preserves the Roman origin, named after Saturn. Many languages use distinct names tied to celestial bodies, deities, or local calendar traditions.

Numerical and programmable representations are common in software and data exchange. Some systems index days starting with Sunday as 0 or 1, while others follow ISO 8601 where Monday is 1 and Sunday is 7. Day-of-week values are often exposed as enumerations in programming languages, sometimes with the first day of the week aligned to Sunday and other times to Monday.

Computing the day of the week for a given date can be performed with algorithms such as Zeller’s congruence or Tomohiko Sakamoto’s method. These techniques enable calendar calculations, scheduling, and validation in software, as well as planning and historical research in broader contexts.