English: One might think that just as one intercalates the year as needed, so one sanctifies the month as needed; therefore it says “this month shall be for you” — see it like this, and sanctify.
On the daf / in context: See the cited source for the full section.
Type of Learning
Derash on the verse (Bavli) — not a formal named middah of the 13.
What Is Learned
The month is sanctified by observation of the new moon as shown, not merely by communal need.