一つ or 1つ -- num.
Hint: if someone said 'hitotsu dake', it means 'only one'
Hint: you say read 日 as hi, ka, and nichi. So for Sunday, you read it as nichiyoubi
Hint: you say Tuesday as kayoubi because 火 can be read as hi and ka.
Hint: Hikari was a popular song for Kingdom Hearts (game), sung by Utada Hikaru