Sorae, you have it backwards. The verb tagen, to hold a Tagung/meeting, comes from the noun Tag = day or parliamentary assembly. How Tag got to mean assembly is more complicated than I realized. From wiki on "diet":
The term (also in the nutritional sense) is derived from Medieval Latin dieta, meaning both "parliamentary assembly" and "daily food allowance", from earlier Latin diaeta transcribing Classical Greek δίαιτα diaita, meaning "way of living", and hence also "diet", "regular (daily) work". Through a false etymology, reflected in the spelling change replacing ae by e, the word came to be associated with Latin dies, "day". The word came to be used in the sense of "an assembly" because of its use for the work of an assembly meeting on a daily basis, and hence for the assembly itself. The association with dies is reflected in the German language use of Tagung (meeting) and -tag (not only meaning "day", as in Montag—i.e. Monday—but also "parliament", "council", or other law-deliberating chamber, as in Bundestag or Reichstag).
English-speaking schoolchildren get a chuckle out of Luther's being summoned to the Diet of Worms.

