Possible Clues:

• Once in this it's not of ten
• Once in, it's most unusual
• Once in it is hardly ever
• Relatively long period of time
• Once in this, that's singularly unusual
• Lunar occurrence (which almost never happens)
• Rare astronomical event
• Rare moment related to rare occurrence
• Beer with an astronomical name
• Classic song performed by Sha Na Na on the 'Grease' soundtrack