Unfortunately the Cygnus DC-8s don't move often, or at least the last three trips I've done to MAD
(last year) they haven't.
The Cubana Il-96 is nice - I caught it on a Monday afternoon and I believe it's thrice-weekly, so perhaps Wed and Fri/Sat too?
As Spencer says, a lot of goodies to look out for, many of which don't appear at other European airports. LAN 767-300s/A340s (difficult to catch if they're using the 33s from the mound), Aerosur 747-300, Pullmantur 747-400s (I remember the 200s/300s in old colours but these have now gone), Avianca 767s/A330s, TAM, Mexicana and Air China A330s, Aeromexico 767s/777s, etc.
European traffic is pretty brisk too, with most major carriers featuring. Iberia heavies can prove difficult if the 33s are in use, but at the mound you're right beside the IB
short-haul ramp so getting IB
's retro and Madrid 2016 jets should be easy enough.
If you do fancy the walk to the 33 approach expect pictures like this...
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Photo © Karl Nixon
Any further info needed, just shout.