You cannot change your DNA.
Spells can work at any time. If the moon is growing (waxing), do positive spells or spells that draw a desired effect to you. If it is getting smaller (waning), do negative spells or spells that send away a certain effect/situation from you.
For instance, let's say u want to attract good luck. If the moon is waxing, you can do a spell to attract good luck, if it is waning, u can do a spell to banish bad luck.
Since you are using the energies of the moon, nighttime is better since you can directly draw it down however it can be done in the day as well, just visualize it happening with your mind's eye.
Hope this helps!