If you planted a tree sapling on firesday, what I would do is feed it fire crystals on firesdays and then harvest it on firesday as close to the full moon as possible --- hoping to grow fire ore.
The main money-maker with the tree saplings is growing elemental ore (which is different from the metallic ores).
Basically from what I've read (mine are still growing) you plant the cuttings in a porcelain pot on the day of the element ore you want to grow, then feed it that same element crystals on the day of that element, and harvest it on the element day, if possible, near a full moon.
It doesn't always work but it will bring in big bucks if it does, depending on the demand for ore on your server. I would check the AH for all the elemental ores (under goldsmithing) before you decide what to go for, and pick the one that's most expensive. Right now I've got 1 wind, 1 dark and 3 lightning cooking hopefully.
You do fail sometimes, so to sort of mitigate my losses, I have one mule just growing saplings (from cuttings, in any old pot, by not feeding any crystals you get 100% success of growing cuttings into saplings) and selling the saplings on the AH for about 3k profit normally. I use any extra that I end up with (you often get 2 saplings from 1 cutting planted) to try to grow ore.
The whole process from cutting to ore takes forever, so any extra gil I get I use to just buy (or use my own) cuttings and start from there.
The Vanadiel Clock does come in handy for this, as it tells you the day of the week it is.
I'll try to remember to post the results after harvest.