occasion在定语从句中作先行词时，后面用关系副词when还是where。

occasion在表示：“a particular time or instance of an event（特定的）时刻；场合”时，定语从句引导词用when；当表示：“a special or noteworthy event, ceremony, or celebration（仪式、庆典等）重大场合”时，定语从句引导词用where。（新牛津英汉双解大词典）

例句：

1. In hard times, the Spring Festival was the only occasion where a family threw off the old attire and put on new clothes specially made for the festival.

2. Describe an occasion when you had to give a presentation to an audience.