From 1966 -- and more relevantly, from Sunday night's penultimate installment of the current season of Mad Men, the one where Don more or less causes Lane to commit suicide...
...please enjoy The Lovin Spoonful (featuring drummer Joe Butler on vocals and the astounding Zal Yanovsky doing guitar versions of pianist Floyd Cramer's coolest licks) --
-- with the gorgeous "Butchie's Tune."
As heard in the closing montage and credits of the episode.
NYMary rightly flagged The Beatles "Tomorrow Never Knows" in an earlier episode as a seriously profound cultural signifier -- and one which cost series writer/creator Matthew Weiner a major part of his budget.
But this Spoonful song (which I assume he got for cheaper) was actually a bigger deal for me. We needn't get into the details at this point, but if you've been around here a while you already know that I've always felt the Spoonful got something of a raw deal when the history of the 60s was written (i.e., they weren't a bubblegum band, not even close).
Which is to say I feel vaguely vindicated that this song underscores perhaps the most appallingly tragic and sad plot point of the show so far.