go up to Jerusalem to worship the King: Jerusalem, with God's temple, is seen as the center of God's worldwide kingdom (Zechariah 8:20–22; see Isaiah 56:6–7; 66:19–23; Micah 4:1; Haggai 2:7; Revelation 21:1–2, 10–27).
the rain: This was one of the curses God announced against Israel for breaking the covenant (see Deuteronomy 28:22–24). That curse also applied to all nations, as God's rule covers all people (compare Zechariah 10:1).