Even I heard that of 21 days, but one of my friend went on 13 Dec and his classes start on 8 Jan and he is in Canada now. So I don't think so there is any problem but make sure you answer well at airport regarding why you came Canada 25 days early.
Applied 2nd time on 26 Nov and refused on 7th Dec (paper based)
MBA from UK
8 year Gap
IELTS_ 7 Band
Applied for PGD Marketing Management @ University Of Winnipeg