際(さい)にWhen, At the time of, In case of, On the occasion of

Both mean at the time of, but differ in nuance. 際に is a formal variant of 時に and applies broadly to past or present, as in 出発の際に写真を撮った (We took a photo at the time of departure). に際して is more formal, limited to present or future occasions, and highlights the special or official nature of the event, as in 契約に際して署名が必要だ (A signature is required at the time of signing a contract).

にあたり・にあたって is used for important occasions that involve preparation or intention, as in 出発にあたり挨拶をした (I gave a greeting on the occasion of departure). 際に simply means when or at the time of, describing something that happens concurrently, as in 出発の際に見送ってくれた (They saw me off at the time of departure).

Both とき and 際に express the time of an action or event, but with different nuances and levels of formality. とき is common in everyday conversation and broadly used, like the English 'when'. 際に is more formal and often appears in official contexts, such as instructions, advice, or warnings related to specific events.

Both ついでに and 際に refer to additional actions tied to another event, but differ in tone. ついでに is an everyday expression for when something is done while convenient, e.g. '買い物のついでに郵便局に寄った' ('I stopped at the post office while shopping'). 際に is more formal and emphasizes a particular occasion, e.g. '来日の際にご連絡ください' ('Please contact us when you come to Japan').

Both mark occasions, but differ in nuance. 折には emphasizes opportunity or timing, often with verbs of judgment or decision, as in 来日の折にはぜひお立ち寄りください (Please visit when you come to Japan). 際に is more neutral, simply indicating when or at the time of, without implying seizing an opportunity.