＊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
chrispy_bear, see [toki] or any other level-3 time expressions.
what is the non-formal version to "ni saishite" ?
You have to keep in mid that 際し is rather formal and would not normally be used in conversation.