Consider that one cycle of the moon phases would be considered as one month. This would mean that in 29 months one month would be over. The 1/2 would not be considered as one day. So, at the end of 12 months the 1/2 days would add up and you would end up with 12×1/2 = 6 extra days.
In the gregorian calender the revoultion around the sun is considered as the year which takes 365.25 days. The months are arranged according to 31 or 30 days. However at the end of 4 years we get one whole day. Hence, the leap has 366 days.
