Jisho

「今年には〇〇の会議が開催されるって聞いたんですがいつです？」I heard they will hold a ** meeting this year, when is the meeting?
「不定期に行われているからまだ日程が決まっていないです」It's an irregular event so the timeline is not yet decided.
=> Mostly confused with 随時, which also can be translated to "irregular" (i.e. not scheduled) but it means "when necessary"
in most case, 随時 is used, cuz 不定期 sounds like an event happened outside of schedule for no reason at all, which is rare. In contrast 随時 means things happened off schedule but for a reason. For example, "I went to hospital for an irregular check up because I was feeling easily tired recently"

地方の市場は、不定期から定期になり、それも回数がふえ、八日、あるいは六日、四日ごとに開かれだした
from 日本の歴史 by 井上清

不定期的ではあるにせよ郵便配達夫も姿を見せるようになった
from 羊をめぐる冒険 by 村上春樹