Apple trees can be planted anytime the ground is not frozen, fall, winter, spring. Planting in summer is about the only time that is NOT a good idea. They don't have separate male and female trees. Every flower has both male and female parts. However, most apple varieties do need to be cross-pollinated by a different variety, some even by two others (these are the triploids, of which there are quite a few). So, you'll want to make sure you have at least two if not three different varieties within about 500 feet of one another so that the local bees and insects can find them all for proper pollination. Otherwise, with just a single apple tree, you could have all the flowers in the world, but zero apples, or apples that don't develop properly.

Have fun planting your tree(s) and enjoying the fruits of labor.

Thank you for answering my question dmtaylor! My sister brought home some apple seeds from school a week ago. I think I'll plant them.

"most apple varieties do need to be cross-pollinated by a different variety, some even by two others (these are the triploids, of which there are quite a few)."

dmtaylor,

Triploid apples are self-sterile but only require one pollenizer to produce fruit.

As an aside, crab apples are generally an excellent pollenizer for apples (they bloom over a long time). Any ornamental crab apples in the neighborhood should take care of pollination (whether triploid or diploid apples).

Apple trees from seeds usually don't produce very good fruit and can take up to 10 years to fruit but it might be a fun project.