Which month is the only month which can not have a full moon? Only the shortest month, February, is capable of passing through all of its days with no full moon. Even though the word month is derived from "moon-th", describing one complete lunar cycle, the length of a month, an actual lunar cycle requires a fractional amount of time greater than 29 days to complete. With only 28 days, or, in a leap year, 29 days, the month of February is shorter than the lunar cycle.
Since the date of the full moon always changes, February can, sometimes, possess a full moon. Sky-watchers should not anticipate its appearance, however. Between 2000 and 2999, a one-thousand year period, 48 years will occur which do not have a full moon in February.
When the full moon fails to appear in February, an event, known as a blue moon, not a reference to the moon's color, happens. Many are familiar with the saying "once in a blue moon", an expression for a rare occurrence. Others have heard song lyrics which state "blue moon, you saw me standing alone". A lyrical event, certainly, the blue moon is the second full moon which occurs in a single calendar month, such as January. If a second full moon occurs, just prior to the onset of February, for example, there simply is not enough time for a full moon to appear in February. Since one thing leads to the next, a blue moon may then also occur in the month following February, known as March, which like January, has 31 days, a necessity for blue moons.
This is what happened in 1999, a year with no full moon in February, which bothered individuals concerned over the lack of a February full moon in the year preceding the onset of the new millennium. Had those individuals known that the blue moons of January and March occur approximately three times each century, they would have realized, as they now know, that their fears were unfounded. Thank goodness these same individuals did not experience the blue moons which appeared in December, 1933 and March, 1934, at the height of the Great Depression, because February, 1934 also had no full moon.
The years in which leap years happen do not ensure that a full moon will shine in February, either. Due to January-March blue moons, three coming leap years will produce no-full-moon Februarys. Don't wait up, though. These no-full-moon events happen in 2572, 2792, and 2944.
A full moon is created by sunlight illuminating one-half of the moon's surface area, an everyday occurrence. The moon appears full to us on earth when the moon and the sun are on opposite sides of the earth. If the sun and the moon happen to be on the same side of the earth, those on planet earth see a new moon, the half with no illumination, which has even spawned its own tribute called the dark side of the moon.