What's the plural form of autumnal equinox? Here's the word you're looking for.
Answer
The noun autumnal equinox can be countable or uncountable.
In more general, commonly used, contexts, the plural form will also be autumnal equinox.
However, in more specific contexts, the plural form can also be autumnal equinoxes e.g. in reference to various types of autumnal equinoxes
or a collection of autumnal equinoxes.
