有一次，我们的老师离开了教室。

On one ___occasion our teacher left the class.

有好几次，我们非正式地开会讨论这个项目。

On several ___occasions， we met informally to discuss the project.

这次聚会对所有参与者来说都是一个欢乐的场合。

The party was a joyous ___occasion for everyone involved.