What's actually happening
A Full Moon happens when the Moon stands opposite the Sun from Earth's point of view. The lunar face is fully illuminated. A Full Moon in Sagittarius means the Moon is in Sagittarius while the Sun is in Gemini — the axis of belief and information, of the general and the particular.
The geometry is one of maximum contrast between the organizing truth and the detail that complicates or confirms it.
What the tradition makes of it
Full Moons are culmination points. What was seeded at the New Moon has enough light on it to be seen. Sagittarius adds a quality of philosophical and spiritual culmination to the moment: the question that has been pursued reaches a provisional answer, the belief that has been tested against experience arrives at something it can honestly hold.
The Gemini Sun has been collecting information. The Sagittarius Moon is asking what it all adds up to. The tension between them clarifies whether the accumulated detail supports the larger vision — or whether the vision has been maintained by ignoring inconvenient particulars.
How to actually use it
Step back from the details accumulated in the preceding weeks and ask what they indicate at a larger scale. This is good weather for the synthesis that has been deferred by the proliferation of new information, and for making the philosophical statement that the evidence has been building toward.
When in doubt
Ask what you believe now that you have the information — and whether what you believe is what you wanted to believe or what the evidence actually supports.