Replying to Keyvan: This used to be a trick that was used.. have only one slot open, and harvest. If you can, you get an ore, if not, the lack of inventory space will block the multiple item harvest, so rinse and repeat. SE nerfed this over a year ago, so now, each attempted harvest with a full inventory counts as an examination, and multiple examinations during harvest season (a) will not change the harvest results, but however (b) will reduce your chances of getting any harvest at all.
Regarding the elementary day: The "elemental theory" (plant, feed and harvest all on the same day as the element you want) will probably build up too much elemental strength in your plants and ruin your chances for a truly superior harvest. I would recommend a harvest on any day except (a) the crystal day, or (b) a day that is strong to the crystal. I would also recommend examining twice a day during the growth period on various mixed days, so long as you do not examine on the day strong to the element used; the element of the day of examining also adds to or adjusts elemental strength of the plant.