I like Joe F's algorithm.

Our USA Mother's Day this year is as usual in May, but it feels more like a rainy April day.

The forecast, however, says that during the coming weekdays, we are due for very warm weather.

I couldn't wait, and got my hair trimmed anyway.
And now I don't think they trimmed enough. But I guess that's better than too much. If they don't trim enough, you can always ask to have more taken off.