The Sage Advice Compendium (pp. 19-20) contains a ruling about the spell moonbeam and other similar spells. The ruling provides a list of spells that are covered by it:
- blade barrier
- cloud of daggers
- Evard’s black tentacles
- sleet storm
- spirit guardians
Whirlwind is not listed. But it does not say that it is a complete list; it merely says that these are "some spells with the same timing as moonbeam for their areas of effect."
The ruling states that (emphasis mine):
Reading the description of any of those spells, you might wonder whether a creature is considered to be entering the spell's area of effect if the area is created on the creature's space. And if the area of effect can be moved—as the beam of moonbeam can—does moving it into a creature's space count as the creature entering the area? Our design intent for such spells is this: a creature enters the area of effect when the creature passes into it. Creating the area of effect on the creature or moving it onto the creature doesn't count. If the creature is still in the area at the start of its turn, it is subjected to the area's effect.
The moonbeam spell description states:
When a creature enters the spell’s area for the first time on a turn or starts its turn there, it is engulfed in ghostly flames that cause searing pain, and it must make a Constitution saving throw.
The whirlwind spell description states (emphasis mine):
A creature must make a Dexterity saving throw the first time on a turn that it enters the whirlwind or that the whirlwind enters its space, including when the whirlwind first appears.
The bolded words are the key difference here. They mean that, unlike with moonbeam, with whirlwind, a saving throw is attempted by any creature within the whirlwind's area of effect immediately when whirlwind is first cast.
But what about when the whirlwind is moved on a subsequent turn? Do creatures attempt the saving throw immediately when the whirlwind is moved onto them? Or, does the Sage Advice Compendium ruling apply, meaning they only attempt the saving throw if and when they are in the whirlwind's area of effect at the start of their next turn?