TAKOBOTO

when; on the occasion of; at (the time of); in; upon (doing something)
When I transferred to Osaka, I left my wife and children behind in Tokyo.

Add formation 

そつぎょうしきにさいして、しちょうからおいわいのことばがそつぎょうせいにおくられた。

Upon their graduation, the mayor sent his congratulatory words to the graduates.

こうじょうけんがくにさいして、かかりのひとからさまざまなちゅういをうけた。

On the occasion of the factory tour, the person in charge laid out various precautions.

そつぎょうにさいして、わたしたちはかんしゃのしるしとしてかれにおきどけいをおくりました。

Upon leaving school, we presented him with a table clock as a token of our thanks.

どくしょにさいしては、かこのいだいなさっかにもっともおおくのちゅういがはらわれなければならない。

When reading, the great writers of the past must be given the most attention.

ごよやくにさいし、クレジットカードのばんごうおよびゆうこうきげんなどのじょうほうがひつようです。

When making your reservation you need to have your credit card number, as well as the expiration date and similar information.

＊The crucial difference between に当たって/に当たり and に歳して is that the former indicates an occasion with a formal sentence, but the latter indicates a special occasion.
＊Therefore, do NOT use に際して when speaking of trivial, everyday occurences.

FORMATION:
Ｎ + に際して

rubyhatchet

You have to keep in mid that 際し is rather formal and would not normally be used in conversation.

bamboo4

what is the non-formal version to "ni saishite" ?

chrispy_bear

chrispy_bear, see [toki] or any other level-3 time expressions.

rubyhatchet