Where does “aestivalis” come from?

aestivalis (Latin) comes from Latin aestīvus, from Latin autumnus, from Latin auctumnus, from Proto-Indo-European h₃ewǵ- — cold.

Ancestry of “aestivalis”, step by step

Step	Language	Word	Meaning
1	Latin	aestīvus	—
2	Latin	autumnus	autumn, fall; autumn; autumnal
3	Latin	auctumnus	autumn; autumn; autumnal
4	Proto-Indo-European	h₃ewǵ-	cold

aestivalis — Etymology, Origin & Word History | Etymology Explorer