Wikipedia
Each month has a flower that symbolizes the month of somebody's birth. The characteristics that the flower has may be "inherited" by whomever is born in that certain month person. Every month has a flower that is sometimes referred to as a Birth Month Flower.
Flowers by month is a term describing flowers related to a recipient's birth month, and in general flowers associated with particular months of the year. It is one of a class of specialized categorizations offered by florists.
In a cultural sense, flower characteristics such as appearance, color, and scent, have relevance as gifts. It is believed that it were the Romans who started celebrating birth and birthdays using flowers. Seasonal flowers were used not just for decoration, but also taken as gifts and therefore can probably be credited with the tradition of birth flowers. Some have been inspired by this tradition to create lists that associate a birthday flower with each of the days in a year.