  We study the semi-classical thermodynamics of two-dimensional de Sitter space
($\text{dS}_{2}$) in Jackiw-Teitelboim (JT) gravity coupled to conformal
matter. We extend the quasi-local formalism of Brown and York to
$\text{dS}_{2}$, where a timelike boundary is introduced in the static patch to
uniquely define conserved charges, including quasi-local energy. The boundary
divides the static patch into two systems, a cosmological system and a black
hole system, the former being unstable under thermal fluctuations while the
latter is stable. A semi-classical quasi-local first law is derived, where the
Gibbons--Hawking entropy is replaced by the generalized entropy. In the
microcanonical ensemble the generalized entropy is stationary. Further, we show
the on-shell Euclidean microcanonical action of a causal diamond in
semi-classical JT gravity equals minus the generalized entropy of the diamond,
hence extremization of the entropy follows from minimizing the action. Thus, we
provide a first principles derivation of the island rule for $U(1)$ symmetric
$\text{dS}_{2}$ backgrounds, without invoking the replica trick. We discuss the
implications of our findings for static patch de Sitter holography.
