I believe the twelve flower seasons is a recent tourist industry initiative in Vietnam, with the idea being that the streets of Hanoi are constantly in bloom with different flowers every month of the year and therefor every month is like a different season. They appear to be the first of a series of stamps with these three stamps showing flowers that are in bloom during January, February and March. Along the top from left to right are sua flowers in March, ban flowers in February and peach blossoms in January.
