Summer is Icumen in

Middle English

Sumer is icumen in,
Lhude sing cuccu!
Groweþ sed and bloweþ med
And springþ þe wde nu,
Sing cuccu!
Awe bleteþ after lomb,
Lhouþ after calue cu.
Bulluc sterteþ, bucke uerteþ,
Murie sing cuccu!
Cuccu, cuccu, wel þu singes cuccu;
Ne swik þu nauer nu.
Pes:
Sing cuccu nu. Sing cuccu.
Sing cuccu. Sing cuccu nu!

Modern English

Summer has arrived,
Loudly sing, Cuckoo!
The seed grows and the meadow
blooms
And the wood springs anew,
Sing, Cuckoo!
The ewe bleats after the lamb
The cow lows after the calf.
The bullock stirs, the stag farts,
Merrily sing, Cuckoo!
Cuckoo, cuckoo, well you sing,
cuckoo;
Don't you ever stop now,
Sing cuckoo now. Sing, Cuckoo.
Sing Cuckoo. Sing cuckoo now!

My favorite version sung by young children:
http://www.youtube.com/watch?v=3-5axHaBlvo&feature=related

I found and listened to several versions of this song.It was always sung in a round with four or more parts.  It is a traditional English round, also known as a rondellus.  This song is believed to have been composed around 1260 and it is not sure who the composer is.

The overall effect of the song is jovial and upbeat which goes along with the theme of the text, which is clearly a joyful celebration of summer.  Almost every performance of this song was sung in a jovial, upbeat manner with the exception of one youtube clip which was the end of a horror movie from 1973 and the people sang it in a slow, creepy manner during a pagan, human sacrifice. This version even added some instrumentation with some minor chords in it to make it a little creepier. But even in the creepiest setting, it didn't really fit, because the melody is so happy sounding.  When sung properly, (not slowly and creepily)  I believe that it exemplifies the "merry sound of England".  Music from England during that time period had specific qualities that was described as a merry sound.  One of the qualities that produces that sound is that many thirds and sixths were present.  This is also what makes it possible to sing in a round with multiple parts.

Side note: The melody and the structure reminds me a lot of "Three blind mice".  In fact I could even sing "Three blind mice"  along with the song and it fit perfectly. But then I noticed that I could also sing other famous rounds along with it, so maybe it is just characteristic of rounds to fit that particular pattern.