1.  occasion  指 “ 时刻；时候；时节；场合” 意义时，定语从句原则用 when 引导。

(An occasion is a time when something happens, or a case of it happening.)

例句：

It was the first occasion when they had both found it possible to keep a rendezvous. 

他们两人都能赴约，这还是头一遭。

来自柯林斯例句

2.  occasion  指“重大活动；仪式；庆典” 意义时，定语从句原则用 where 引导。

（An occasion is an important event, ceremony, or celebration.)

例句：

The birth of a New Year is always a special occasion where people gather to celebrate a new beginning brimming with hope and renewal . 

新年历来是特别喜庆的日子，人们欢度新的开始，新的希望。

【注】如果第二句的主语改为 New Year' Eve，那么 occasion 就表示“时间”，而不是“事件”了，那么关系副词改用 when：

New Year' Eve is always a special occasion when people gather...