Uriel’s Machine may be used once by a player to counteract any Very Special card. If the current date is an Equinox or Solstice date, then the player is allowed to keep and use the opponent’s Very Special card a second time. May be used at any time!
If players have access to current and accurate astrological data for the year in which they are playing the game then, by all means, use that data to determine the precise Equinox and Solstice dates (hyperlink above). If not, then simply use these approximate dates: