Which rate CRA expects
For capital gains, CRA's guide T4037 says that a foreign amount should generally be converted at the Bank of Canada rate in effect on the day of the transaction. It also lists conditions under which CRA generally accepts a rate from another published source. For Form T1135, the instructions use the rate at the time of the transaction for most amounts, the average for the year for maximum funds and maximum fair market value, and the rate at the end of the year for year-end values.
Weekends, holidays and older dates
The Bank publishes one rate per business day. For a weekend or holiday, this page shows the latest earlier rate and says which date it came from. The daily series used here starts on 2017-01-03 for most currencies. A few have shorter runs or gaps, so the page shows the range for the currency picked and says when no rate is close to the date. For dates before 2017, the Bank keeps older noon and closing rates on its site. They come from an older method, so the two series are not the same measure.
What these rates are
The Bank of Canada describes its exchange rates as indicative: averages of transaction prices and quotes from financial institutions, published for statistical and analytical use. They are not the rates a bank or broker would give on a real conversion. A broker's own statement may show a different rate for the same day.
Use them in the ACB calculator and the T1135 calculator, which fill in these rates by date.