Fifty days after Easter — the descent of the Holy Spirit on the apostles.
Pentecost falls 49 days after Easter Sunday and commemorates the descent of the Holy Spirit on Jesus's apostles, as described in the Acts of the Apostles. It is regarded as the birth of the Christian Church and is the third-most important festival in the Christian calendar after Easter and Christmas.
Computed for Bois-Guillaume (Europe/Paris) using local sunrise, sunset and astronomical positions.