aestival
Concise Oxford English Dictionary © 2008 Oxford University Press:
aestival /ˈiːstɪv(ə)l, iːˈstʌɪv(ə)l, ɛˈstʌɪv(ə)l/ (US also estival)
▶adjective technical belonging to or appearing in summer.
– origin ME: from L. aestivalis, from aestivus, from aestus ‘heat’.
'aestival' also found in these Oxford entries:

