Esbats mark the change of the moon, the same way Sabbats mark the change of the seasons. Where most Judaeo-Christian beliefs have their members gather once a week, Witches tend to gather at least once a month. Most times, these gatherings, or Esbats, are on or near the Full Moon.
Now, having said that, there are always exceptions. Esbats are the rituals where Witches do their magick. Because Witches believe the phase of the moon has a huge impact on that magick, different types of magick are done on different phases of the moon.
The Waxing Moon is the two weeks where the moon is growing from New to Full. Waxing Moon magick is done to draw things to the Witch. If for example, you need money for rent, you might do a Prosperity or Money Spell during the Waxing Moon
Most Witches are familiar with the Full Moon Ritual. It is the most commonly held public type of ritual. As the moon waxes from New to Full, the power of the magick also grows. That makes the Full Moon the most powerful time to do magick to draw things to you.
As the moon moves past Full, it is now Waning for two weeks toward the Dark Moon. As the Waxing Moon was for magick to draw things to you, the Waning Moon is for magick to push things away from you. You want to quit smoking, or lose weight? The Waning Moon is the time to do these types of spells.
Many new Witches have a fear of Dark Moon Magick. They hear tales of dark magick rituals done at the Dark Moon. Really, the Dark Moon is simply the polar opposite of the Full Moon. As the Full Moon was the most powerful time to draw things to you, the Dark Moon is the most powerful time to push things away.
Just after the Dark Moon, there is a day or so where the moon is new. There is no beginning or end of the Lunar Cycle, it is constant, just like the Wheel of the Year. The New Moon is used to mark a start of the Lunar Cycle, just as January 1st marks the start of the new year.
The New Moon is used to start spells that will take a full two weeks to manifest, or for some spells, you will be spell casting each night for the entire month.