What's actually happening
Mercury's orbital period is 88 days, but its apparent motion through the zodiac from Earth is uneven. During direct motion, it spends roughly two to four weeks in each sign. Aquarius is a fixed air sign, traditionally ruled by Saturn with Uranus as a modern co-ruler.
What the tradition makes of it
Mercury in Aquarius is the mind as theorist. The interest is in the underlying structure — the rule that explains the exceptions, the pattern that holds across different cases, the idea that generates other ideas. Individual cases are interesting mainly as instances of something larger.
The tradition notes this placement's affinity for original thinking. Mercury in Aquarius is not easily impressed by convention, and its willingness to question what is assumed creates space for genuinely new insight. The best thinking under this sky is often counterintuitive.
The shadow is the preference for the abstract over the lived. A system that works in theory must eventually be tested against the actual. The individual case the theory discards is often the one that matters most.
How to actually use it
Follow the unconventional thread. This is good weather for theory, for thought experiments, for the kind of conversation that proceeds from "what if the premise is wrong." Let the thinking be strange if it wants to be.
The practice is to bring it back to earth before publishing. Originality is not the same as accuracy.
When in doubt
What would the system look like if the standard assumption were removed?