After the slow summer period passed, business picked up.
Literal
Recession-like summer period [subject-が] passed [when-と], business [topic-は] became-lively.
不景気な marks the summer as economically slow. 活気づいた = 'became lively/bustling' (活気 + づく). ~と here is the temporal/consequence conditional: 'when X happened, Y followed.'