際に【さいに】

際に means "when", "on the occasion of". It is the formal word for とき時に, and you meet it in notices, instructions, manuals and business messages. A verb goes in the dictionary form or the た-form; a noun takes の.

The noun is almost always an action – applying, leaving, using – and in polite notices it carries お or ご: ごりよう利用のさい際, おかえりお帰りのさい際.

際に or 時に

とき時 goes with anything, including a state or a stretch of life. 際 marks one particular occasion, so it doesn't follow a state, an い-adjective or a period like childhood.

• × 子供の際に → 子供の時に
• × 忙しい際に → 忙しい時に

In conversation with friends, 際 sounds stiff. Use とき時 there.

際は

With は in place of に, the occasion becomes the topic: "if or when that happens". Signs and instructions use it for what to do in a given situation.

そのさい際 points back to an occasion already mentioned: "at that time".