This is a bit of an easy question to answer yet it is not so easy. In a nutshell, the constellations can be seen any night and at any time! As long as it is night time and the skies are clear. If you are seeing stars then you are seeing constellations. The reason for this is that the complete night sky is filled with constellations. Every area of the sky, all night and through the while year has constellations that you can find.
The following picture explains this a bit. The earth is in its current seasonal place in its orbit around the sun. It will take a year to go all the way around the sun and get to the same place. So you can see by the picture that some of the constellations won't be seen at this time of the year (the sun is in the way!). But all those constellations on the other side of the earth (away from the sun) can be seen every night.
So this means that only some of the constellations can be seen at any time of the year.
But when it comes to finding particular constellations in the night sky this is the real trick of the whole situation. The earth is rotating so you are going to only see some of the constellations at one point in the night and later in the night as the earth has rotated you will get a different view of the night sky. The following two drawings show this well. This is the real secret of seeing the right constellations and until you get to recognize the constellations by memory it really helps to have a small rotating star chart.
This challenge in finding the constellations is easily overcome with something called a star wheel. You simply turn the dial to the current month and day and time of night and you get a pretty accurate look at what stars and constellations are currently out.
The Night Sky is a rotating star finder- or planisphere that allows the user to recognize the constellations for any time of night, any day of the year. The sky appears to rotate (due to the rotation and orbital motion of the earth), so to be successful recognizing the constellations a beginner needs to know which stars are above the horizon at any time.
This is the full-sized version of The Night Sky suitable for the 40°-50° latitude zone (northern United States and parts of Europe). We have versions for the following latitude zones: 50°-60°, 40°-50°, 30°-40°, 20°-30°, and the Southern Hemisphere. We also have pocket-sized versions available for the same latitude zones.