The timing of spells and rituals is crucial. You can draw on the prevailing energies of the moon, the sun and the seasons and also factors like weather to amplify the power of your spells. To flow with the energies of the natural world is like walking or cycling with the wind or swimming with the tide as opposed to against it.
Much of the information on the moon, sun, seasons, the sea tides and weather is contained within different books. However, successful timing are ultimately about trusting your innate instincts. A number of experienced practitioners can feel if the moon is right or sense a rising tide even before they reach the shore.
This is not so easy initially for city dwellers or for people who like me grew up in the center of towns. The more time we spend close to nature, the more this instinctive awareness returns, for we all carry it in our genes from our distant ancestors, though it may be deeply buried.
You can help to reinstate this awareness by rising and sleeping according to natural light patterns. This is easiest done on holidays or weekends especially on natural camping grounds where there is no electric light. Watching the moon in the sky without light pollution is a good way of connecting with natural rhythms and timing especially if you note down your feelings as the moon changes each night. You may find a particular moon day evokes the same emotions each month.
I still have a tendency to think I should switch on the heater on a grey day even if it is muggy but I am getting better at listening to my body and not my logical mind.
Again, successful circle casting is less a matter of accurate measurement and knowledge of correct procedure than a feeling the pulse of the land you are working with and allowing your instincts to help you create an empowered and protective area for magick.