For many decades, the rule when selling stocks on the market was that “settlement” — actual completion of the sale — would happen in three days. This rule came into place long before there was computerized trading and electronic delivery of shares.
Since September 5, 2017, settlement of a share purchase or sale now takes place in two business days. This change applies to markets in both Canada and the U.S.
This affects year-end selling done for tax purposes. If you are selling shares to trigger a capital loss to use for 2017, or to trigger a capital gain to use up other losses, make sure to finalize your trade no later than December 27, 2017. Since December 28 and 29 are Thursday-Friday, your trade will then “settle” on December 29. As the following two dates are Saturday and Sunday, a trade on December 28 will not settle until January 1, and will not count for your 2017 tax return.