Or even have an "Endless Summer".
Endless summer is metaphorical at best. You can swap hemispheres, but at the end of one summer you still end up in the spring of the other.
I don't think the concept is about fixed dates on a calender. It's about being somewhere warm, year-round. And you can set an itinerary such that you can accomplish that, by migrating from country to country. and always being in the right place at the right time. It may be "spring" somewhere, but it can still be warm enough to enjoy summer activities.
To Nigel: Glad you had that neat experience. It seems to be getting pooh-poohed here as no big deal, but it actually is. Very few people get to experience such an event because of the things required to make it happen, and good on you for having the gumption to do it,