George Gershwin wrote ‘Summertime’ (lyrics by DuBose Hayward) as an aria to the 1935 opera “Porgy and Bess”.  This song has subsequently been recorded over 2,600 times and is unofficially tied with the Beatles’ single "Yesterday" as the most popular song of all time.  (http://www.answerbag.com/q_view/860618)

Its whimsical nature can be traced to the inspiration for the song which comes from a Ukrainian lullaby “Oy Khodyt Son Kolo Vikon”.

Ukrainian Translated into English Transliterated into English
Ой ходить сон, коло вікон.
А дрімота коло плота.
Питається сон дрімоти:
"Де ж ми будем ночувати?"Де хатонька теплесенька,
Де дитина малесенька,
Туди підем ночувати,
І дитинку колисати.

Там ми будем спочивати,
І дитинку присипляти:
Спати, спати, соколятко,
Спати, спати, голуб’ятко.[1]

 The Dream passes by the window,
And Sleep by the fence.
The Dream asks Sleep:
"Where should we rest tonight?"Where the house is warm,
Where the child is small,
There we will go,
And Rock the child to sleep.

There we will sleep,
and will rock the child:
Sleep, sleep, my little falcon,
Sleep, sleep, my little dove.

 Oy khodyt’ son, kolo vikon.
A drimota kolo plota.
Pytayetsya son drimoty:
De zh my budem nochuvaty?De khaton’ka teplesen’ka,
De dytyna malesen’ka,
Tudy pidem nochuvaty
I dytynu kolysaty.

Tam budem spochyvaty,
I dytynku prysypl’yaty:
Spaty, spaty, sokol’yatko,
Spaty, spaty, holubyatko.

Gershwin wrote it after hearing "Oy Khodyt Son Kolo Vikon" at a 1926 concert of the Ukrainian National Chorus under Oleksander Koshetz, in New York City.
(lyrics and facts courtesy of : http://en.wikipedia.org/wiki/Oi_Khodyt_Son_Kolo_Vikon)

Here is a sample of the original lullaby:

Porgy and Bess was intended to be an "American Folk Opera" revealing the plight of African Americans in America.  Its centerpiece aria was inspired by a Ukrainian lullaby and the rhapsodized by a Jew.  This is a telling summary of the fundamental power of music.

Here is a close representation of what the original was intended to sound like as scored:

 

Here is a more popularized version, highly popular at that:  Louis Armstrong and Ella Fitzgerald

 

All of the divas of the day recorded this tune so let’s take Ella’s rendition as representative.

Artie Shaw delivers a defining Big Band rendition:

 

Here is a creole style jazz rendition of Summertime by Sidney Bechet.  This recording and subsequent album launched his career.

 

Starting to diverge more significantly from the original, Stan Getz offers a bassa nova stylized rendition:

 

Miles Davis renders a more interpretive jazz version which became highly popular in the 50’s

 

Janis Joplin recorded a version in the late 60’s which emphasizes the soulful nature of the piece.