Donations to be receipted for a year must be RECEIVED by the charity in the same year, and that receipt must be evidenced.
If you mail a receipt or date a cheque in a year and mail it late in the year, but the mail is received in the new year, we can only receipt the item for the prior year if the return address is on the envelope and a postmark on the envelope must show that it was mailed in the prior year. If the postmark is not there, or legible, then the item cannot be receipted for the prior year. It is not uncommon for the postmark not be there or dated.
To ensure any donations in the last few weeks of December are receipted for that year, instead of mailing a donation the best option is to donate online via www.CanadaHelps.org