Once a bottle is opened and the wine is exposed to air, it begins to deteriorate. This chemical process is known as “Oxidation”. If a bottle is consumed shortly after opening, then oxidation isn’t really an issue. If it sits around for more than a few days the wine may become dull & stale and perhaps even undrinkable.
There are products available to fight the oxidation process. For example there is a non-toxic spray that removes all of the oxygen from opened wine bottles. Also, wine sold in boxes, though some consider it gauche, is much less suceptible to this problem because the wine is not exposed, is well sealed and has a tap.
Just call some friends to help finish your wine and you won’t have an issue. Empty bottle, no oxidation, no problem.