The first time or the first session (e.g., the first meeting, first exam).
第一回の会議は来週月曜日に開かれます。
The first meeting will be held next Monday.
彼女は第一回の試験で合格した。
She passed on the first attempt.
この企画は第一回から好評を博している。
This project has been well received since its first edition.
Meaning 'first' or 'initial' used as a noun phrase; can appear in nominal constructions (e.g., 'the first of ...').
第一回参加者には記念品が贈られる。
First-time participants will receive a commemorative gift.
大会の第一回開催は1998年だった。
The first staging of the tournament was in 1998.