Related על יומא 123:6

What does it mean "half [פרס]" before Passover and "half [פרס]" before Pentecost and "half [פרס]" before of Tabernacles? R. Yose the son of R. Yehuda says: Not less than fifteen days before the [relevant] pilgrimage festival. And why is it written on the baskets [which contain the shekels in the temple] "Alef, Beit, Gimel"? They write "Alef, Beit, Gimel" on it, because they started with the first and bring out of it. When the first was empty, you bring out from the second. When the second was empty, you bring out from the third. When all three were empty, and the time came to separate the [next] appropriation, one gives from the new, but if not, from the old one.