The grape harvest likely refers to the time when a new vineyard produces its first crop, about four years after planting. The owner would receive grapes or wine as payment for leasing the vineyard.
In the Old Testament, prophets are often referred to as servants (Jeremiah 7:25–26; Amos 3:7; Zechariah 1:6). Israelite leaders frequently treated them badly and killed them (1 Kings 18:13; 19:10, 14; 2 Chronicles 24:20–27; 36:15–16; Jeremiah 26:20–23; Nehemiah 9:26; see Matthew 23:34, 37; 1 Thessalonians 2:15).