on the occasion of; at the time of; when
Formal expression used to introduce the occasion or time when something happens. Often used in speeches, announcements, and formal writing. Similar to に際して and にあたって.
On the occasion of the opening, I would like to say a few words.
At the start of the new year, I wish you all good health.
に当たって has the same meaning and formality as にあたり, but にあたり is the continuative form and feels slightly more literary.
にあたり is a terser, more literary variant of にあたっては, commonly seen in written notices and reports rather than spoken language.
Derived from the continuative form of 当たる (あたる) with the particle に, literally 'in hitting/coming upon (a time/event)'. The expression became a fixed formal phrase.