There are a number of different configurations for the Council of Ministers, each corresponding to a policy area.
Do these configurations gather ad-hoc or on rotation?
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Sign up to join this communityIt might depends on the configuration.
ECOFIN (Economic and Financial), which is one of the oldest (configurations) generally meets once a month. (Likewise for AGRIFISH and Foreign Affairs.)
COMPET, which is newer, meets (at least) four times a year. Likewise for the Environment (ENV) and JHA (Justice and Home Affairs).
TTE (Transport, Telecommunications and Energy) even though it's theoretically one config, in practice it's three:
The composition of the TTE Council and the number of its meetings varies according to the items on the agenda:
transport ministers usually meet four times a year
energy ministers meet three or four times a year
telecommunications ministers meet twice a year
The precise composition of the Council depends on the items discussed in a particular meeting. [...] The EYCS Council meets three or four times a year; twice in its full configuration.
Etc. I'm not sure there's one that meets entirely ad-hoc. It looks like twice a year is the minimum frequency and once a month is the maximum for regular meetings of some arbitrary config.