应该是on special，on special是固定搭配on special: 以特价出售例句与用法: 1. I only wear a tie on special occasions. 我只在特殊场合才打领带。

2. I only wear high heels on special occasions. 我只在特殊场合穿高跟鞋。

3. Coffee is on special (ie being sold at a lower price than usual) this week. 本周咖啡大减价.。