on the occasion of; at the time of
Expresses 'on the occasion of' or 'at the time of' a formal event or action.
に際して is used to talk about the time or occasion of a significant, often formal event. It has a written, formal flavor and appears in announcements, speeches, and official documents. It is not used in casual conversation. When attaching to a verb, it must be in its dictionary form; when attaching to a noun, the noun directly precedes に際して. The pattern can be used to describe something that happens before or during the event, or as a condition for something else.