For multi-city flights, how can I find the cheapest flights if my dates are flexible?

I need Skyscanner's calendar view and chart view for multi-city searches, but Skyscanner appears to offer them only for one-way or return trips.

E.g.: My dates are flexible as follows; so how do I determine which dates offer the cheapest flights?
1st Leg can be flown on any date in June: City 1 → City 2
2nd Leg can be flown on any date in September: City 2 → City 3
3rd Leg can be flown on any date in October: City 3 → City 4

I already tried the common search engines per these articles of Lifehacker and of Michael Bluejay.

flight-search-engines

Best Answer

Not sure if there are tools that will get you the result directly in an easily comprehensible form, but if not, the procedure to do so "manually" would go as follows (I'm considering you're actually coming back to the departure city):

Find out the cheapest fare for the whole trip. You can use tools such as ExpertFlyer to look up fares.

Note that in you case, it will need to be a fare from your origin to one of the two "destinations", which allows routing via the other destination, as well as at least one stopover. This will quickly eliminate a lot of the cheaper fares. You may also have a few other rules (validity dates, min/max duration...) that may further restrict which fares are applicable.
Once you have found the valid fares, check out the associated booking class for each (the first letter of the fare code).
For each of the flights, check for availability in those classes (for that airline) over a range of dates. Again, tools such as Expertflyer will let you do this. Once you've found a sequence of flights available with the right class, you should be able to book the whole trip on those dates. If there are no availabilities, start again with the next higher fare.

Note that you may also want to consider booking the flights independently (or a combination of two tickets). In some cases, you might even save by taking a series of return flights even if you don't use the return.

All in all, quite a bit of work indeed.

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Where can I find cheap flights with open dates?

Click here to search on Skyscanner. Skyscanner lets you see a month of fares at a time, which can save you a bundle if your dates are flexible. It will also let you see the cheapest month. To see flights for a whole month, or choose the cheapest month, just click in the Depart box.

Are flexible flights cheaper?

The flexibility to travel on another day can result in dramatically lower fares. So next time you search on KAYAK, select the flexible dates option. You'll probably find that flying Saturday is cheaper (though not so much for international).

Is it cheaper to do multi-city flights?

A multi-city flight combines flights between several cities into one reservation, saving you the time of booking several one-way tickets between each location. Often, it's a lot cheaper than booking a series of one-way tickets.

HOW TO FIND CHEAP FLIGHTS - My Best Tips After Booking 500+ Flights

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: John Diez, Lennart Wittstock, Tim Gouw, Julia Volk