Talking About Your Actions

に [ni] is a particle that means 'in', 'at', 'on', or 'during'

休みの日に何をしますか [yasumi no hini nani o shimasu ka]
What do you do on your day off?

• 休みの日 [yasumi no hi] means "day off"
• に [ni] in this case means "on"
• 何をしますか [nani o shimasu ka] means "What do you do?"
• Literally means "Day off, on, what do you do?"

誕生日に何をしますか [tanjōbi ni nani o shimasu ka]
What do you do on your birthday?

• 誕生日 [tanjōbi] means "birthday"
• Literally means "Birthday, on, what do you do?"

Most words that name a time of day, rather than a day itself, don't pair with -に [ni].

朝、何をしますか [asa, nani o shimasu ka]
What do you do in the morning?

• 朝 [asa] means "morning"
• 何をしますか [nani o shimasu ka] means "What do you do?"
• Literally means "Morning, what do you do?"

昼、運動します [hiru, undōshimasu]
I exercise in the daytime.

• 昼 [hiru] means "daytime"
• 運動します [undōshimasu] means "I work out"
• Literally means "Daytime, I work out"

• A: 朝、何をしますか
  What do you do in the morning?
• B: 朝、散歩します [asa, sanposhimasu]
  I take a walk in the morning.
• A: あ〜。休みの日に何をしますか
  Oh, what do you do on your days off?
• B: 休みの日にアニメを見ます [yasumi no hini anime o mimasu]
  I watch anime on my days off.
• B: 休みの日に何をしますか [yasumi no hini nani o shimasu ka]
  What do you do on your days off?
• A: 休みの日にショッピングします
  I go shopping on my days off.