(Exodus 23:16) "And the festival of the harvest (Shavuoth) the (time of bringing the) first-fruits of your labor": The three festivals are mentioned in the section of shevi'ith to teach that (in the shemitah year) the three festivals are not to be dislodged from their proper times (though these times correspond to those of planting, harvesting, etc., operations which are suspended in the shemitah year).