I opened a wine on a Tuesday and didn't like it. I retried the opened bottle three days later and it was awesome. Doesn't wine get worse after it's opened?
I have two theories for your happy discovery. Wine does, indeed, tend to bite the dust a few days after it's been exposed to air. But the rate of descent depends on the style. Some wines, notably such full-bodied reds as cabernet sauvignon, can blossom before they wither, like a bowl of chili or stew that tastes better the day after it's made. Astringent tannins will soften and the fruit will step out of the shadows. (Lots of metaphors, I know.) That's the main reason you might want to decant a big, young red prior to serving.
The other possibility is that you simply enjoy the taste of (overly) oxidized wine. Fresh plum and berry flavours tend to transform into prune and raisin after one to three days. Some people, though not I, are fond of such bruised-fruit notes even though certain connoisseurs would pronounce the wine down for the count.
Years ago a reader wrote me about an experience similar to yours. What he said was revealing. The dry red wine had acquired a sweet taste similar to port, he said – with an enthusiasm that suggested he'd just transmuted lead into gold. Port often suggests prune, though I would argue it achieves a better balance between acids, sugars, alcohol and tannins than does a dry wine that has simply gone off. Each to his or her own.