Conversation so far:
where do the names of the days of the week come from
The names of the days of the week in many languages are derived from the names of the classical planets in Hellenistic astrology.
where do the days of the week come from in other cultures
In some other languages, the days are named after corresponding deities of the regional culture, either beginning with Sunday or with Monday.

Question: which is the first day
Self-contained rewrite: which is the first day of the week