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Wherever the Common Era Calendar (a.k.a. the Gregorian Calendar) is used — and it is now used by the governments of all countries — a week of seven days is also used in conjunction with it. But there is no 7‑day cycle in Nature from which this could have been derived, so why a week of seven days?
People use a 7‑day week because they have been born into a world where this is customary. In other words, the 7‑day week has been received from earlier generations. It has a long history. When the Roman emperor Constantine made Christianity the state religion early in the 4th Century CE the 7‑day week was officially associated with the Julian Calendar, and the association remained after the Julian Calendar was replaced by the Gregorian Calendar in the 16th Century CE.
The Christians received the 7‑day week from the Jews (in fact, the original Christians were Jews). The Jewish explanation for its use is that this was commanded by their god, named by them YHWH (using the Hebrew letters Yod-He-Vav-He). The Jewish Pentateuch (incorporated into the Old Testament of the Christian Bible) contains several injunctions attributed to YHWH which mention "a seventh day", upon which no "work" is to be done.
Whether or not a 7‑day week was in use by the Jews at the time of Moses in the middle of the 2nd millennium BCE is highly debatable, since YHWH's commands to Moses are not preserved in any contemporary records but only in documents which were composed around the middle of the first millennium BCE.
