A polite, somewhat formal expression referring to a specific past or future occasion previously mentioned. Often used in business or formal correspondence.
Thank you very much for your help at that time.
I look forward to your continued support on that occasion as well.
Standard modern spelling.
Archaic kanji form using 其の; rarely used in modern writing.
Combination of その (that) and 節 (occasion, time). The use of 節 to mean 'time' or 'occasion' is a formal, somewhat literary extension of its basic meaning 'joint' or 'segment'.