Simplify the following text for a Grade 8 native English-speaking person.
Original: The thirteen hui ã å 3\/4 1\/2 ã on the surface represent the 13 months of the year ( the extra 13th is the ` leap month ' in the lunar calendar ) .
Simplified: The thirteen hui on the surface represent the 13 months of the year ( the extra 13th is the ` leap month ' in the lunar calendar ) .