際に details
write + (past) + upon doing
kaita saini
upon doing; on the occasion of — a notable event or juncture
meaning: upon writing
念のため、出発したさいに、忘れ物がないかご確認ください。
nen no tame, shuppatsushita saini, wasuremono ga nai ka go kakunin kudasai.
As a reminder, upon departing, please check that you haven't left anything behind. formal
帰国した際に、この書類を提出してください。
kikoku shita sai ni, kono shorui o teishutsu shite kudasai.
Upon returning to your home country, please submit this document. formal
Examples
upon doing; on the occasion of — marks a notable event or formal juncture as the setting for the main clause, often an instruction or requirement
念のため、出発したさいに、忘れ物がないかご確認ください。
nen no tame, shuppatsushitasaini, wasuremono ga nai ka go kakunin kudasai.
As a reminder, upon departing, please check that you haven't left anything behind. formal
帰国した際に、この書類を提出してください。
kikoku shita sai ni, kono shorui o teishutsu shite kudasai.
Upon returning to your home country, please submit this document. formal
何か問題が起きた際に、担当者に伝えてください。
nanika mondai ga okita sai ni, tantousha ni tsutaete kudasai.
If any problem comes up, please let the person in charge know. neutral
ご予約の際に、お名前とお電話番号を伺います。
go yoyaku no sai ni, o namae to o denwa bangou o ukagaimasu.
When you make a reservation, we will ask for your name and phone number. formal
Formation
{V-ta}さいにattaches to た-formplain clause (dict verb / i-adj / な-adj) + 際にattaches to plain clause{N}のさいにattaches to noun
Notes
この際 "at this juncture" — a fixed idiom, not the pattern's ordinary use.
Related patterns
Similar
際に is とき's formal, written-leaning register partner and additionally narrows to a notable or official juncture (契約の際に, ご来店の際に) — bare とき tolerates any moment at all, trivial or not, and is what casual speech actually uses in the same slot.
Lookalikes
Both readings open with さい and both are formal に-clauses, but 際に marks a single notable OCCASION that need not be ongoing at all (契約の際に), while 最中に specifically requires an activity ALREADY IN PROGRESS at its peak (会議の最中に) — swap them and either an ongoing activity gets invented that isn't there, or the interruption reading disappears.