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.