Pool opening and closing times are primarily dependent on the area you live in. Some states such as Florida don't even know what openings and closings are, since they've never had to do it. However, for the rest of us it's par for the course.
In our area, east Tennessee, weather is anything but predictable. That being said, some people wait too long to open their pool and / or close their pool too soon.
We average around 150 pool openings and 150 closings per year. The best time to open your pool is before the 80 degree days hit. March to Mid-April. Some people wait until May to open their pools. These people almost always have a green mess on their hands and spend 2 to 3 times more on getting their pool nice and clear. That is expensive.
I tell everyone: Even though you're not swimming, it's much cheaper to open it March to Mid April and have the pump run a few hours a day and add some chlorine here and there then to wait too late and spend a fortune in cleanup.
The same goes for closing your pool: don't close it too early just because you're done swimming for the season. Wait until these 80 degree days are over - then close the pool. October to November is the best time to close. If you do it too early, your water is susceptible to an algae outbreak. Plus, you don't want that much water sitting around for 6,7, or 8 months. Your closing chemical effectiveness is dramatically reduced after 5 months of your pool just sitting there. Plus if you have a mesh cover like many ingrounds, rain water dilutes the chemicals even quicker.
Timing your openings and closings correctly will save you a fortune and prevent unwanted hassles that tie up weekend after weekend doing cleanups. Use your head: open early, run the pump a little each day, and enjoy a pretty pool when the warm days return. Close your pool when you see those 80 degree days are gone for the year - usually October and November.
Have a great swim season everyone!