ここは普通(ふつう)の漁村(ぎょそん)だが、夏休(なつやす)みともなると大勢(おおぜい)の客(きゃく)が来(き)て、賑(にぎ)やかになります。
This is an ordinary fishing village, but when the summer vacation starts, a large number of visitors come and it becomes lively.
AがBともなるとC is a very common pattern expressing what happens (C) when B (some special circumstances like an emergency) happens. A is often an ordinary situation, or current stage in life, etc.