This is the first line and title of a traditional English hymn, commonly sung at harvest festivals. Jon hums this to himself to calm his nerves. That he does so unconsciously is indicative of how deeply entrenched his Christian upbringing is in him, despite his claims to having shaken off the faith.
A Christian feast celebrated 50 days after Easter Sunday. It commemorates the descent of the Holy Spirit upon the followers of Jesus. It is from this Biblical occurrence that the Pentecostal movement derives its name. The Monday following Pentecost Sunday is a public holiday in many European countries.