There are two main factors for when the perfect time to close your pool is...

1. Temperature
  1. Climate plays a big role. If you live in a warm climate year-round, you may never close your pool.
  2. Water Temp: You can close your pool once the temperature of the water remains consistently at 60°F (16 C) or slightly below. Algae cannot grow below that temperature.
  3. Holidays: According to a survey of pool companies in the US, Most residential pools stay open from Memorial Day to Labor Day.
  4. Pool heater? Keeping it open? Inground pool with an attached spa?

2. Swimming
  1. Back to school and kids aren't using it
  2. Too cold to swim
  3. Just doesn't feel like pool season anymore.

If you close too early while it's still warm, you run a higher risk of opening to a lot of pool algae. The cover and stagnant warm water is the perfect environment for algae to grow.

If you close too late, you run the risk of freezing and damaging equipment. But as long as the water is moving, it won't freeze. So keep that in mind.

Ideally, when the water gets to 60 degrees (16 C) and no one is swimming anymore. That tends to be around the middle to late September of course depending on where you live.