It is the registration date determining it the course is eligible for tax credit!!!
Check this:
"You can claim the children’s fitness tax credit for each child who was, at the beginning of the year in which the registration or membership fee was paid, under 16 years of age or under 18 years of age and eligible for the disability tax credit (as long as another person has not already claimed the same eligible fitness expenses and that the total amount claimed is not more than the maximum that would be allowed if only one of you were claiming the credit). "
"Eligible fitness expenses include amounts paid by you or your spouse or common-law partner in the year, regardless of when the activity takes place. Amounts paid before 2007 do not qualify for the credit."