Pesach has Matza, the Seder, the four cups of wine, "Ma Nishtana.…" Succot has the Four Species and the Succah. Shavuot, however, has no single identifying Mitzvah, no recognizable landmark in its scenery. Why?
Nineteen hundred years ago, the twenty-five thousand pupils of Rebbe Akiva all died in the days of the Omer between Pesach and Shavuot. This was a punishment for not treating each other with enough respect. The majority of the days Omer are in the month of Iyar. Thus Iyar is a time for us to work on improving respect for others.