Who knows twenty-seven?
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27 se'ah of coins from the annual half-shekel donations were designated as "terumas halishkah" - the funds to be drawn on for the regular operating expenses of the Beis Hamikdash. (Any funds collected in excess of those were called "sheyarei halishkah," and were used only if expenses were higher than normal, or for exceptional projects.) (Rambam, Hil. Shekalim 2:4ff)
27 are the letters (including final forms).
27 (and a piece) are the days of a revolution of the Moon around the Earth by any way of measuring, other than by how it lines up with the Sun).
How the current 29-day (and a piece) Molad cycle (which approximates the Sun-based or Synodic month - the period between full moons or between new moons) lines up with the current 27-day (and a piece) Anomalisitc and Tropical months accounts for a great deal of the variability in the former. According to Alex Schutz in "Di Shmaya: Of the Heavens," this is a likely explanation for the Rabban Gamliel's tradition that "Sometimes [the moon] comes on a long path, and sometimes in comes on a short path" (Rosh Hashana 25a).