Google flights[1] kind of does this. If you put in an itinerary, add whatever other constraints you have (e.g. airline, flight times, fare class), and then click the calendar and wait a second, it'll eventually populate with prices under each day for changing the itinerary to that day. The cheapest day is colored in green so it jumps out.

It's not perfect, the calendar view shows two months at a time, and at times it's easier to use as starting point, but it's definitely better than clicking through 100 itineraries on an airlines slow site.

[1]: https://www.google.com/flights