Yes, 35-40 years ago the networks often did play those movies on the appropriate days. I remember it well. But not so much since the advent of cable and satellite television. And hardly at all in the last 20, even less since then.
I was speaking to the more recent past. For the last 15 years or so, TCM and AMC have been playing those movies, at least on Memorial Day weekend, and on the weekend closest to December 7. They played mostly standard fare again this year, but this year they did skip "The Longest Day" last weekend, when normally they play it that weekend every year. Given that I had always said they should hold it for a day closer to June 6, if not on June 6, I had high hopes. But I started hunting for it and checking IMDB Friday, and found it wasn't going to run. that's a shame.
TCM and AMC seem to hold the rights to most of the good movies now. And of course the networks couldn't care less about history, being too busy with reality shows and that ilk.
However, recenty, both TCM and AMC have been cutting their marathons short, as they did this year. One ran theirs from Friday to Sunday night, the other from Sunday to Monday. One of them also did something for Eastwood's 80th, and played "Kelly's Heroes" and "Where Eagles Dare", both classics, and standard fare for the weekend. I do appreciate what AMC and TCM run, but it's not enough to suit those of us of the hardcore bunch.
It seems that CMT, or whatever they call it, has been airing "Band of Brothers" almost exclusively lately.