Thinking about a flaw appropriate for a Cultist of Mercury / Neo-Mercurian. What would an appropriate astrological period be when a Mercurian would be at a disadvantage? My first thought is when Mercury is in retrograde, but I have no idea if that is a period appropriate concept.
Would that work? Is there some other astrologically significant period when a Mercurian would be at a disadvantage?
Most astrological concepts date back to antiquity, so that's not a problem.
However, the times when Mercury is in retrograde are too short and too far between for it to be valid for the flaw. Cyclic Magic need to give a bonus/flaw for an equal amount of time as when it doesn't.
There are no obvious periods that would be relevant for Mercurians, so just pick something.
I would recommend having the flaw apply for two out of the four seasons of the year - that way you will at least have two seasons available for lab work without the flaw applying.
Mysteries revised does have rules for Cyclic Magic with an uneven period - so this is mechanically possible. Doesn’t mean it’s a good idea 
If you want to tie the cyclic magic to mercury there are some less common possibilities- for example when mercury is in an earth or water sign. Technically you could base it on signs mercury is in and keep it fairly seasonal- mercury is rarely far from the sun so you could easily have an arrangement which would leave one season where it applies, one where it doesn't, and two where it applies part of the time.
Part of the issue becomes how much effort you want to put into tracking astrological events from 900 years ago.
Outside the age of aries? No, nevermind.