Grammar Practice: Choosing the Right Words for Time and Occasion

I can never forget the day 'which' we worked together and the day 'when' we spent together.

Explanation:

The word 'which' is used to refer to a specific day out of many possibilities, while 'when' is used to refer to a specific time or occasion. In this sentence, 'the day' refers to a specific day, and 'we worked together' and 'we spent together' refer to specific occasions. Therefore, the correct choice is 'which' to refer to the specific day, and 'when' to refer to the specific occasions.