最中に details
write + (and…) + right in the middle of — an intrusion striking an activity at its peak
kaite irusaichuuni
right in the middle of — an intrusion striking an activity at its peak
meaning: in the middle of writing
ちょうど電話しているさいちゅうに、誰か来たんだよね。
choudo denwashite irusaichuuni, dareka kita n da yo ne.
So, in the middle of talking on the phone, someone showed up, you know. casual
会議の最中に、携帯が鳴ってしまった。
kaigi no saichuu ni, keitai ga natte shimatta.
Right in the middle of the meeting, my phone went off. neutral
Examples
right in the middle of — an intrusion strikes while the marked activity is at its peak
ちょうど電話しているさいちゅうに、誰か来たんだよね。
choudo denwashiteirusaichuuni, dareka kita n da yo ne.
So, in the middle of talking on the phone, someone showed up, you know. casual
会議の最中に、携帯が鳴ってしまった。
kaigi no saichuu ni, keitai ga natte shimatta.
Right in the middle of the meeting, my phone went off. neutral
ただいま契約手続きの最中に、システム障害が発生いたしました。
tadaima keiyaku tetsuzuki no saichuu ni, shisutemu shougai ga hassei itashimashita.
Right in the middle of the contract procedure, a system failure occurred. formal
Formation
{V-te}いるさいちゅうにattaches to て-form{N}のさいちゅうにattaches to noun
Notes
電話しているさいちゅうに works; 最中 alone (最中だ) also stands as its own
predicate outside this に-clause frame.
Related patterns
Similar
あいだに just places the main event somewhere inside a neutral interval with defined ends (会議のあいだに); 最中に insists the main event is an intrusion striking while the marked activity is AT ITS PEAK, and reads naturally with a sudden interruption (電話が鳴る、来客がある) where あいだに would feel flat.
うちに frames its window as closing, something the speaker must beat before it lapses (暗くならないうちに); 最中に makes no closing-window claim at all — it only names the peak of an ongoing activity being interrupted, with zero urgency to finish anything before a deadline.
Lookalikes
Both readings open with さい and both are formal に-clauses, but 際に marks a single notable OCCASION that need not be ongoing at all (契約の際に), while 最中に specifically requires an activity ALREADY IN PROGRESS at its peak (会議の最中に) — swap them and either an ongoing activity gets invented that isn't there, or the interruption reading disappears.