I agree with Winterleafs point about doing your research on the phases of the moon, there is actually a brilliant article for this on the site which is the first link below, the second one below that is about working with spells and the moon, which will help to aid you in determaning what kind of moon phase you need to be in for your spell to be effective.
Also it would be very beneficial to prepare your spell in advance of the actual night you plan to conduct your spell on, If I may I'd like to help give you some tips to try to increase the chances of your spell being timed correctly.
Spells are energies that can combine any number of the elements Earth, Air, Fire, Water & Spirit, We use spells in prayers, in poems and stories even when we make a wish or some kind of intention we are sending out an encoded message to the universe or the matrices of energy that intertwine all things. However most of the time people who are not aware of the mechanics involved often see now results at all.
This is due to the fact that our energies are not imbalance with that it is we are trying to achieve, furthermore some people even email me to say they feel possessed by demons and negative energies, and this is why I always ask if they had prepared before doing anything.Preparation and planning all helps towards not only having a chance at succeeding in your task, but more importantly that it was conducted with safety in mind. If you consider that it's not just energy your directing, but your affecting other energies too, it's a very busy world out there so you need to make sure your aware of other things too.
Firstly cleanse your environment with a simple air cleansing spell, recite mantra's or even physically clean your personal workspace. That way you have a clean place and a clean mind for it to reside within. Now we can consider casting a circle. But before we do that, are your magical objects cleansed as well, make sure you have gotten rid of any excess energies form these too, also ground yourself and get rid of any excess energies that may be residing in you just before you cast. Now we can move onto the actual casting of the circle.
So not only is your environment clean, your magical objects you'll be working with and your inner energies, you are also safely in the protection zone of your circle. If you are working with dark magic or demonic magic, make sure you have made all the preparations for that too, although even with the best defences working with the forces of darkness is never a safe endeavour.
Also one very comforting fact is that the more time you spend preparing, planning and researching your spell, the greater the chances of you being safe and your spell being a success! Magic is all around us and is a very fine art to master, but the journey is the most fun part about it. If you make safety your main priority and practise meditation on a daily basis to keep your energy at a healthy optimum level, then you'll definatly increase the chances of your spell being timed correctly as you are becoming more inbalance with that it is that your intending.
Hope this helps you, Good luck! :)