Photoperiodism is the change in a plant's behavior in response to changes in day length
In the 1920's it was discovered that the seasonal production of flowers in plants is based upon the length of the available daylight -- that is, the plants actually measure the length of the day in order to know when to bloom.
Subsequent experimentation showed that blooming behavior could be artificially influenced by manipulating a plant's exposure to light. In some experiments, a plant that was kept in darkness and exposed to only momentary flashes of light could be induced to bloom or not bloom depending on the timing of the light flashes and the wavelength of light used. Recent work as shown that, in terms of photoperiodism, plants are only sensitive to light at 660nm and 730nm wavelengths.
So how does a plant actually measure light?
Plant cells contain many biologically active light-sensitive compounds, called photopigments, (the most well known example is chlorophyl.) A certain type of photopigment, named simply photochrome, is converted back and forth between two distinct forms depending on its exposure to 660nm (present during daylight) or 730nm (present at or near sunset) light. It has been shown that when photochrome is exposed to 660nm light, it can move into the plant cell nucleus and cause changes in gene expression -- which can lead to the changes in blooming behavior.
(For the biology geeks: 660-converted photochrome translocates to the nucleus where it binds to phytochrome-interacting factor 3, (PIF3), a a helix-loop-helix bearing transcription factor which recognizes and binds to the sequence 5'-CACGTG-3')
730nm light inactivates the 660nm form of photochrome and thus stops any changes in gene expression due to exposure to 660nm light. In some plants, the role of the two forms may be reversed -- that is, 730nm-converted photochrome may play the active role, and 660nm light may deactivate it.