you say a few days? Like how many?
It's been my experience that if you open a wine and leave it, it gets worse...not better. Of course it depends on how many days.
It could be the wine is getting a chance to "breathe". They sell decanturs to speed up this process. I have no idea what this does, or why it makes wine better, but you swirl it in this large glass pitcher and it supposedly makes it taste better.
Other than that I am clueless
<----is of the open it and drink it till it's gone class