The New moon is when the moon is all dark. The crescent moon is when it looks like a “C” shapes. The Quarter moon is when half of the moon is visible. A Gibbous moon is between the quarter and full moon stages. The full moon is when the moon is all lit up. A second full moon in one month is called a blue moon. read more
We could, perhaps, name a different phase for every day since full moon. But we don’t. Arbitrarily people pick 2, or 4, or 8, depending upon the application and upon the precision required. read more
However, the Moon can appear to be full a day before or after while more than 98% of the Moon's disc is illuminated. When a Full Moon occasionally passes through Earth's shadow, it will cause a lunar eclipse. read more