Choosing when to cast a spell need not be complicated. If you feel like casting a spell, go to it. However, many people feel that spells are more powerful when they are cast during certain moon phases and moon signs and even during specific days of the week. This is because various planetary energies are believed to be more available for use during this time and those energies can add a bit of strength to your own.
There are many options for spell timing listed here. Please do not feel that you need to time your spell to coincide with all of these options. Any one is helpful. You can layer them if you like, but it is not necessary.