際（さい） is a noun in Japanese that means ‘verge’. This is as in ‘to be on the verge’, and translates mostly as ‘on the occasion of (A)’, ‘at the time of (A)’, ‘in case of (A)’, or ‘when (A)’. In the same way as ‘verge’ in English, it implies a fairly big or somehow important event, and will not be used with standard occurrences.
際（さい）に will appear after verbs in either their dictionary or past-tense forms, and after nouns. In the case of nouns, it will be preceded by の.
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この建物（たてもの）に入（はい）る際（さい）には、必（かなら）ずヘルメットを被（かぶ）ってください。
When entering this building, please wear a helmet.
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この薬品（やくひん）が誤（あやま）って肌（はだ）に飛（と）び散（ち）った際（さい）には、直（す）ぐにかかりつけの医者（いしゃ）に相談（そうだん）してください。
If this chemical accidentally splashes onto your skin, consult your physician immediately.
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明日（あした）の面接（めんせつ）の際（さい）には、印鑑（いんかん）を持（も）ってきてください。
Please bring your stamp when you come to the interview tomorrow.
While (A) is likely to be a temporary or one-off event,
際（さい）に is not limited to one-off situations. It may also appear with regularly occurring events, but the way of reacting to that event may change depending on some one-off circumstances.
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商品（しょうひん）を並（なら）べる際（さい）に、注意（ちゅうい）しないといけない事（こと）はなんですか。
Is there anything I should be careful about when arranging the products?
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車（くるま）を動（うご）かす際（さい）には、周（まわ）りに子供（こども）がいないか確認（かくにん）してください。
When moving your car, please make sure there aren't any kids around.
Caution - Although
際（さい）に and
時（とき）に may overlap in certain situations, if 時（とき）に is expressing something that usually never changes,
際（さい）に will sound unnatural.
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日本（にほん）では家（いえ）を出（で）る時（とき）に行（い）ってきますと言（い）う。
In Japan we say, ‘I’m leaving,’ when we leave the house.
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日本（にほん）では家（いえ）を出（で）る際（さい）に行（い）ってきますと言（い）う。
In Japan we say, ‘I’m leaving,’ when we leave the house.