You can choose any 12-month period for your medical expense claim, as long as it ends in the tax year. Set your claim period to get the most out of your medical expenses:
https://turbotax.community.intuit.ca/replies/3259119
Consider adjusting your prior year return to claim older expenses that don't fit well within your chosen claim period for this year's return. This is how to adjust a prior year return: https://turbotax.community.intuit.ca/replies/2647992