"on which days" is correct and usable in written English.
You can use it to ask for clarification about a specific time period, e.g. "What days of the week do you work on?" "On which days?".
There came endless lists of edicts that Jews, Muslims, and Christians still impose on themselves: what one was not allowed to do, on which days, with what consequences if one failed.
This includes rules about personal space, who does which chores on which days, how long you can borrow one another's things for, and everything in between.
On which days do museums have reduced fees?
Members can see things like who will be traveling on which days.
"You can easily find the thrice-weekly Paris-Moscow Express in the 'Cook Book', for example, but without already knowing on which days it runs, you've only a three in seven chance of finding it using a journey planner".
Muslim arrestees were frequently asked to reveal the mosque where they prayed, and on which days; the schools their children attended; the airline they took when they arrived in America; how often they returned to their homeland; the jobs and addresses of family members back home; whether they'd gone on pilgrimage to Mecca or fasted on Ramadan.
Depending on which day your race lands, avoid running or working out at least one to two days before the race so your body can rest, repair and rejuvenate.
On which day did the mammoth get created?
It really does depend which team turns up on which day.
It is not known on which day Goldman lost money, but the loss was $25 million to $50 million.
Who did what to whom on which day? Who said that?
