on the occasion of; at the time of; when doing
Formal expression used before a noun or verb to mark a special occasion or the start of an action. Often used in speeches, announcements, and formal writing. The related form に当たり is more formal and often used in written language.
See also: にあたり
On the occasion of the opening, I would like to say a few words of greeting.
新しい事業を始めるに当たって、しっかりとした計画が必要だ。
When starting a new business, a solid plan is necessary.
際に is a more neutral, general-purpose equivalent usable in both casual and formal speech, while に当たって is reserved for formal or ceremonial contexts.
にあたって and にあたっては mean the same thing, but the は in にあたっては emphasizes the introduced matter as the topic or condition under discussion.
に際して and にあたって are nearly interchangeable formal expressions meaning 'on the occasion of,' though に際して leans slightly more toward emphasizing the timing of the event.
に当たって marks the moment of undertaking an action ('when doing X'), while にあっては marks the surrounding situation or circumstance ('in such a situation'), so they aren't interchangeable despite both sounding formal.
Derived from the verb 当たる (to hit, to correspond) in its te-form, combined with the particle に. The expression literally means 'in hitting/corresponding to' and developed into a formal marker for occasions.