This is a big one: Summertime is probably amongst the "all-time top 5" Jazz standards in history. It's a timeless tune, played and recorded over and over, that stood the test of time. It was composed by George Gershwin in 1935 for the American folk opera Porgy and Bess with lyrics by DuBose Heyward.
It's a very deep and meaningful piece of music. I often perform it in "cocktail" situations. It works very well with a backbeat on drums. Singers often call Summertime (unexpectedly) at jam sessions: Could you play it in the "singer's key"... ?
...that's like an unwritten rule of jazz! Accomodate the singer or sit this one out. (-:
So, my main suggestion for jazz guitarists here is: practice this tune in all 12 keys! It's worth the effort and it's not that hard! If you're somewhat familiar with jazz harmony, you'll notice that the two main focal points of this tune are Dm and Gm ... towards the end there's an Fmaj chord, which is in fact the *relative* major. So in very general terms: it's in in a minor key, it goes to iv minor and then to the relative major.
Don't memorize it "chord by chord": see the big picture and think of it as 4 or 8 bar sections. Sing the melody along, you'll be able to play it even in the "hardest" keys. Enjoy!
Here's the chord melody PDF (a very easy version). The PDF also contains a basic chord sheet for accompaniment (comping.)
Miles Davis' album Porgy and Bess recorded in 1958. It features a brilliant orchestral arrangement of this tune (by Gil Evans.) Definitely in my top 50 jazz albums of all time!