Do I always have to say 「来週の」when talking about a date next week?


I will use an English example as a sentence. Let's say today is Thursday.

> What day are you going shopping next week?  
>  I'm going shopping on Sunday.

Which is the correct way to answer in Japanese?  
来週は何曜日に買い物に行きますか？

Is it:

> 日曜日に買い物に行きます。

Or:

> 来週の日曜日に買い物に行きます。




In your case, the topic has been already fixated to 来週(は) in the question, so
you don't have to say 来週 again. Everyone understands you are currently talking
about the next week's plan.

If there were no previous context, 日曜日に買い物に行きます normally refers to 次の日曜日 (
_next_ Sunday), which can be different from 来週の日曜日 (Sunday of the next week)
because many think a 日曜日 is at the end of a week. For example, when 次の日曜日
refers to 3 days from now, 来週の日曜日 may refer to 10 days from now.

