に際して|にさいして

On the occasion of; at the time of. Marks an event, procedure, or undertaking as the occasion for a formal action such as a greeting, thanks, announcement, or notice.

A formal expression found in speeches, ceremonies, official notices, business documents, and letters. It frames ceremonial occasions such as openings, graduations, departures, or contracts, and in formal customer notices it also frames ordinary procedures such as shopping or using a service. In speeches it is often followed by humble expressions like 申し上げます. Compared with 際に, it sounds more formal and ceremonial.

• にあたってN2「にあたって」 has the same attachment and register as 「に際して」, and the two are interchangeable in speeches (e.g. 「開会に際して／にあたって」); however, 「にあたって」 often also introduces preparations made for the event, while 「に際して」 focuses on the moment itself.
• にあたりN2「にあたり」 is equally formal and also attaches to nouns or the dictionary form of verbs; in speeches, as in 「開会にあたり」, it can be swapped with 「に際して」. It more often introduces preparations or statements of intent for the event, and is mostly used for planned undertakings.
• 際にN3「際に」 only indicates the timing of an action and carries no nuance of a major event, so it works in everyday formal contexts such as procedural explanations. It attaches to nouns with 「の」, and can also take the た form to refer to past events. For major occasions, the two are mostly interchangeable.
• 〜に先立ってN2「〜に先立って」 means that preparations, explanations, ceremonies, etc. are carried out before the event begins, i.e. at a point earlier than the event, whereas 「に際して」 refers to the moment itself. When modifying a noun, the former uses 「に先立つ＋noun」 and the latter uses 「に際しての」.
• 折にN1「折に」 broadly refers to some occasion or opportunity, not limited to major events; it can take the た form to describe past events, and is often used as 「お/ご〜の折には」 to express invitations or wishes, frequently seen in letter greetings. Invitations like 「お越しの折には」 cannot be replaced with 「に際して」.
• をきっかけにN2「をきっかけに」 emphasizes that the preceding item is the trigger for a change or new action, with a neutral register; it attaches to verbs with 「の」 and can use the た form. For example, 「結婚に際して転居した」 can also be said with 「をきっかけに」, but the emphasis shifts to the cause.

にあたって/にあたり are close in meaning and formality but tend to stress facing an important undertaking, while 際に is a more neutral 'when', and 折に sounds softer and more polite, often appearing in letters. に先立って means 'prior to' the event rather than at it, and をきっかけに marks a trigger that leads to some change rather than simply the occasion.