I did not come up with the following, but surely believe that there is truth in it.

The first Spring harvest is the barley harvest which would represent the pretribulation rapture.

Barley is harvested by winnowing... tossing the grain up in the air to separate the grain from the husk.

The next harvest is the wheat harvest which could very well represent a prewrath rapture.

Wheat in ancient times was harvested using a tribulum (a word we get tribulation from)!

The tribulum was a board embedded with stones to bruise the kernels of wheat to separate it from the chaff.

Lastly comes the harvest of grapes.  At the second coming, Christ will trod the grapes in the winepress.

I found these similarities to be profoundly thought provoking.  I look for the pretrib rapture in the Spring and rejoice believing that at long last He is at the door.