[verb (dictionary form)] + にあたって / にあたり [noun] + にあたって / にあたり [noun] + にあたっての + [noun]
にあたって means "on the occasion of": in the run-up to something important, someone prepares, decides or says something. It follows a noun or a verb in the dictionary form, and it is formal, at home in speeches, notices and business writing.
The verb stays in the dictionary form even when the whole sentence is past, because the occasion was still ahead when the preparing happened.
The occasion has to be a big one: a new job, a move, an opening, a contract. An everyday act sounds absurd with it.
にあたり is the same pattern in a stiffer register, typical of official notices and written greetings. にあたりまして is the polite spoken version for speeches.