Which term refers to something related to each day or that takes place daily?

The term that refers to something related to each day or that takes place daily is "diurnal." This word is derived from Latin, where "diurnus" means "daily." In various contexts, it is often used to describe events or phenomena that recur every day, such as daily variations in temperature or patterns of animal behavior that are influenced by the daylight cycle.

In contrast, "periodic" generally refers to events that occur at regular intervals but not necessarily daily, which could be weekly, monthly, or at any set timeframe. The term "weekly" specifically denotes occurrences that take place once a week, while "sequential" pertains to events that follow a particular order or sequence, without any indication of daily repetition. Therefore, diurnal is the most accurate descriptor for daily occurrences.