Forsythia and a raft of other plants, flower heavily in the spring. When the spring bloom flowers fall off, that is when you prune. Don't feel bad. There are, actually, very few people out there that know how to prune properly.
You should always have three sizes of stick.. one, two, and three year old wood. Imagine your thumb, your index finger, and your baby finger. (not actual sizes, just examples of scale). After the bloom, cut out all the three year old wood. Then the two year wood becomes three; one year wood becomes two; and new shoots that come out of the ground become first year wood. I have found that for every two sticks you cut out, you generally get three or so new shoots coming up.
What your goal should be is to have long, slender, gentle arching branches, that come up from the ground. They will grow to the actual height the plant is supposed to attain, and stop! You never prune or cut off branches at the end. If a particular branch offends you, for whatever reason, cut it off completely within roughly two inches (5 cm) of the ground. If you cut the end to trim it, the latent buds will sprout and you can end up with an octopus like cluster of growth. Each of these new branches say to each other, "hey guys , we're supposed to be four feet high (120 cm). Lets go." Now you are fighting a branch that is trying to be eight feet tall. You get the picture?
When you use this technique to prune, you will get flowers from the bottom, to the top of the sticks, every year.
If you don't, the three year old wood, and older, will bear few, if any flowers. there is no such thing as periodic pruning. This should be done every year, if you want a show piece.
There are a raft of plants (flowering and none flowering) that should be done like this. Wegielia and dogwood are just two other common examples. Cheers