when are the preliminary matches held in naruto?

The preliminary matches in Naruto are held after the second phase of the Chūnin Exams, when too many genin advance and a one-on-one elimination round is needed to cut the field down. In the main Chūnin Exams arc, the preliminaries happen before the finals, and the finalists get about a month to recover and prepare.

When they happen

• They are not a fixed seasonal event inside the story; they happen as needed when the exam has too many survivors after the second round.

• In the original Chūnin Exams arc, the preliminaries are held right after the second stage and before the final tournament.

• After the prelims, there is a gap of one month before the final round.

Simple answer

If you mean “when in the exam,” the preliminaries are held between the second and final stages of the Chūnin Exams. If you mean “how often,” they happen only when enough candidates remain to make a full elimination round necessary.