- During the first seven months of the Zoroastrian calendar year, that is from mah Farvardin to mah Meher, all the five Gehs are recited according to their appropriate timings. However, during the last five months of the year, that is from mah Avan to mah Asfandad and the five Gatha days, Rapithwin geh is not recited and instead of that Hāvan Geh is recited again. This Hāvan geh, recited instead of Rapithwin geh is called the ‘Biji Havan’ or “the second Havan” geh.
- This practice had commenced in Iran thousands of years ago when a religious calendar, which started around March, was followed along with the state calendar. In this religious calendar, the last five months of the year were winter months when the days were too short.
- On account of short days and late sunrise, it was difficult to perform rituals like the Ijashni, which are specific to Havan geh. Hence the Rapithwin geh was added on to the Havan geh to facilitate the performance of rituals.
- There was another reason too for not praying Rapithwin geh during this period. It was believed that Rapithwin Yazad, who is the Yazad of mid-day and hence also of warmth, had to go into the core of the earth to give her warmth during the winter months and hence should not be regularly invoked during these months.
Ramiyar Karanjia 
so according to shehansai calendar biji havan starts sometime in the month of March and it keeps changing. It does not make sense. So during summer time we should pray rapithwan geh and not Havan geh and knowingly still we do the wrong thing. Fasli calendar witch starts from March 21st is the true Zoroastrian calendar and should be followed.
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All the high priests and scholars come together and resolve this calendar issue once and for all.
Please note spelling error in previous comment: WHICH instead of witch.
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