This is the 8715th most frequent Japanese word.
時に
"時に" means "at times" or "sometimes" in English.
In this sentence, '時に' (時には) is used to express a specific time or condition when being busy necessitates taking a break.
忙しい時には、休むことが大切です。
When you are busy, taking a break is important.
Here, '時に' specifies a particular point in time when the subject was a student and certain experiences occurred.
学生だった時に、日本へ旅行しました。
When I was a student, I traveled to Japan.
In this instance, '時に' defines a specific event or circumstance during which the action of forgetting the umbrella took place.