サイ, きわ, ぎわ, わ
occasion, side, edge, verge, dangerous, adventurous, indecent, time, when
JLPT3 G5 S14 F183
As per 祭# (scrape) + 阜 piled earth → scrape earth in creating a structure → *come into contact* → *edge* (← point of contact for the lines of a wall or other construction) → *brink* → *occasion* (← point of contact).
[Full Kanji Etymology List]