Most people choose the destination first and the dates second. That is backwards, and it is the most expensive mistake in trip planning.
The same country can cost twice as much and be half as pleasant six weeks apart. Roads close. Ferries stop. None of that shows up in a photograph.
How the strip works
Every destination guide carries this near the top:
JanFebMarAprMayJunJulAugSepOctNovDec
BestShoulderAvoid
Best means weather and price agree. Shoulder means cheaper with a real risk attached. Avoid means crowds, heat, monsoon, or a price spike that is not worth it.