Postponements are a series of man-made rules not found in the Bible but are used by the ruling priesthood to establish the high day Sabbaths for the coming year. It was because they did not have a recorded, running calendar which began at creation, showing all the dates for the special Sabbaths. Therefore, there was no accountability for the days that they set.
The rules used were very involved, especially the one requiring the sighting of the moon. This was done to establish the authority of the priesthood. It wasn’t until a brilliant Jewish bookkeeper presented a calendar in 1838, based on mathematical calculations, that the postponement rules were no longer required. This math is the basis of the calendar presented here.