If this was referring to a day of the week with an e ending, e.g. "Ili iros al Francio dimanĉe, I understand it would normally be taken to be referring to multiple Sundays, and would be translated as "They will go to France on Sundays". Does that convention apply only to days, but not to other units of time such as months or, as in this case, seasons?
While you are generally correct, not all speakers make this distinction, and even those who do (like me) will sometimes deviate from it for stylistic reasons.
In this particular example, context tells us that this is a future event - so the intended meaning either has to be that they will go this coming spring, or that they will start a new habit of going every spring. I personally find the first meaning more plausible.