Jewish Babylonian Talmud – Tracts Shekalmi.
p. 12 CHAPTER III. MISHNA: (a) At three periods of the year money is drawn from the treasury (of the Shekalim); viz.: Half a month before Passover, half a month before Pentecost, and half a month before the Feast of Booths. The same dates are also the terms for the obligation of cattle-tithing, so says R. Aqiba.