Translation:We will go swimming tomorrow and the day after tomorrow.
Because the verb is "to go swimming" (which isn't all that different than "to swim"). "We are going to go swimming" is correct, but sounds a little strange (too much go), so "We will go swimming" sounds better here
when talking about activities, the verb to go should be paired with the gerund (verb with -ing ending used as a noun).
- "I go shopping", not "I go to shop".
- "he went hiking", not "he went to hike".
- "we will go swimming", not "we will go to swim".
If purpose is emphasized, the ifinitive is the common idiom. "I go to play, not work." "I am going to see the sights." (If "am going" is used in the sense of simple futurity the gerund is usual, but, if the sense of actually "going" is intended, the ifinitive is common.)
I talked about this topic with my aunt from Vietnam recently and she told me 'ngày kia' means 'the day after tomorrow' as well.