Glad you asked...
A lunar eclipse can occur at very different frequencies. For a lunar eclipse to happen, the planets are required to be in line a certain way. Due to the requirements, a lunar eclipse can happen sporadically. Normally, they happen in a group and then stop for awhile and then happen again. You can find more information here:
http://www.... souledout. org/nightsky/eclipsefrequency/eclipsefrequency. html
