にあたり

Means 'on the occasion of' or 'at the outset of' an important event or undertaking. When facing a significant moment such as an opening, a new year, or a new project, the speaker does something fitting, such as greeting the audience, stating intentions, or preparing.

A formal expression common in speeches, official greetings, announcements, and written documents. It marks a notable starting point or turning point, such as an opening ceremony, a new fiscal year, or a project launch, and the main clause states a fitting action like a greeting or a statement of resolve. It is slightly stiffer than にあたって and more ceremonial than 際に, which is also fairly formal but can describe ordinary situations.

• にあたってN2「にあたって」is the て-form of 「にあたる」and has the same meaning as 「にあたり」, so the two are generally interchangeable. Both are formal, but 「にあたって」is less stiff, and it can also modify a noun as 「にあたっての＋noun」.
• に際してN2「に際して」is just as formal and likewise attaches to nouns or the dictionary form of verbs; the two are often interchangeable in speeches and announcements. It stresses the moment itself rather than preparation or attitude, and in writing it can appear as 「に際し」, with 「に際しての」used to modify nouns.
• 際にN3「際に」simply marks a timing and is not limited to planned undertakings — it also works for sudden situations and for looking back on the past. Add 「の」after a noun, and verbs may also take the た-form; it is often interchangeable in formal notices, though speeches and formal statements favor 「にあたり」.
• に先立ちN2「に先立ち」emphasizes doing something 'before' an event and can also mean doing it ahead of other people or things, with the following clause able to describe a situation that arises beforehand. 「にあたり」, by contrast, refers to the moment of undertaking something; the two are often interchangeable when talking about advance preparations or giving a speech.
• 折にN1「折に」broadly refers to an occasion or opportunity and is not limited to the start of something; it can take the た-form to describe what happened at the time, and it often appears as 「お/ご〜の折には」to invite someone to visit. Because 「にあたり」focuses on undertaking something, the two are generally not interchangeable.
• を機にN1「を機に」treats the preceding event as a turning point, with the following clause describing an action or change that begins from it, whereas 「にあたり」refers to the occasion of welcoming or undertaking something and is typically followed by speeches, preparations, or explanations. With verbs it must be phrased 「〜のを機に」, and the two are generally not interchangeable.

にあたって has the same meaning but is slightly less stiff, and に際して is similarly formal but focuses on the occasion itself. 際に is also relatively formal but can describe ordinary situations, に先立ち stresses doing something before an event, 折に refers politely to an opportunity or time when something happens, and を機に means taking an event as a chance to start something new.