In most part of the country, the ceremony takes place in sept-oct (before the rainy season). There are still communities who do it at other time but it is now illegal for health considerations. At least this is what I learned from my seven trip there. However I have never covered it.
When do you plan to go? I might go back in mid-june.
I know a good driver. His name is Liva.
He knows the country very well and he is reliable.
But he speak french. You can reach for him at this email:
rgn@mel.wanadoo.mg
I have done my Ilakaka (saphirre town) reportage with him.