I do a lot of my gardening according to the moon's phases. The theory is that as the moon influences the water around us, so it can aid the growth (or not) of plants, or any other living thing. The waxing moon is the time for sowing and planting stuff that produces above the ground and the waning moon for root crops and onions.
There are of course a few more elements to lunar planting like whether the moon is in Aries (good for fruit) or Taurus (good for roots) etc.
I was planning on sowing celeriac, beetroot and onion seed today but as it is a leo fruit day and a full moon to boot, I shall leave it and transplant some toms instead! :)