The fourth month of the year according to the Shire Calendar, following Rethe the third month, and followed in turn by Thrimidge the fifth month. Astron was roughly equivalent to modern April, but not exactly so: in modern terms it ran from 23 March (soon after the Spring Equinox) to 21 April.
The structure of the Shire Calendar was developed from the Kings' Reckoning of the Dúnedain, and so the thirty days of Astron map directly onto the equivalent month from that calendar of Men, which they called Víressë or Gwirith. The name also varied in Bree, where the same period of thirty days was given the name 'Chithing'.