What does John 4:35 mean?
In this passage, Jesus has drawn an analogy between the "spiritual food" of serving God and the physical food which refreshes the body (John 4:34). Here, He continues that analogy by comparing the work of evangelism to that of collecting crops.The phrase Jesus uses at the start of this verse might have been a local proverb. Experienced farmers could look at the season and know how far away the harvest-time was. Those same farmers could look at their fields to recognize the right time to bring in the crops. Similarly, the disciples should have the ability to recognize the right time for a spiritual harvest.
In this case, the Samaritan woman is bringing many people from town back to meet Jesus (John 4:30). When a wheat field is ready to be collected, the tips of the plants appear white. The crowd of people, in white garments and headwear, might have echoed this same imagery. Presumably, the disciples could have turned from Jesus to see the sight of an approaching crowd and would have grasped His meaning. Evangelism is not merely something to be done when we think it's convenient; waiting until the perfect moment means never getting anything done at all (Ecclesiastes 11:4).