Fairly early (1400's? or maybe before I think). And, although everybody knows the first line, the rest is tricky stuff.
This is by the way a round - though not often sung that way.
Summer is acumen in
Summer is acumen in
Loude sing cuckoo
Groweth seed and bloweth mead
And spring the woods anew
Sing cuckoo
Ewe now bleateth after lamb low
After calf the cow
Bullock starteth, buck now verteth
Merry sing cuckoo
Cuckoo, cuckoo
Well sing'st thou cuckoo
Now cease than never now