English: "Observe..." — "Observe the month of Aviv"—observe the Aviv of the solar season so that it falls in the month of Nisan.
On the daf / in context: The calendar must be intercalated when necessary so that the spring equinox coincides with the month in which Pesach is celebrated.
Type of Learning
Derash on the verse (Sifrei) — the requirement to keep Pesach in the spring.
What Is Learned
The month of Nisan must be kept in the spring season; the calendar is intercalated when necessary to achieve this.