thanks for this! 11 years from the first conference is actually a better number than 10 years ... and then 22, 33, and so on ...
just to mention from my own schedule that beginning of october or end of september would be better for me and my students. usually we have donaueschingen festival in mid of october, and the semester has just started. and if i recall correctly, end of september were most of the previous conferences.
but of course it is about you and your institution to decide.