To find the total fiat (USD/CAD/EUR) value of your balances on a particular date:
2. Find the prices for the cryptocurrencies you held on that date.
You can use the closing price on the daily charts on Kraken Pro for this.
Please note that Kraken Pro does not include exchange rates between two fiat currencies. So if you need the exchange rate for your EUR balance in USD (for example), you'll need to find that information outside of Kraken.
3. Multiply each cryptocurrency balance by the relevant fiat price.
4. Add up the fiat values.
It will help to organize your work on a spreadsheet application, as shown below:
IMPORTANT: Countries differ on how cryptocurrency transactions, trades, and holdings are taxed (if at all) and how they view cryptocurrencies in general (e.g. as money, as property, as a commodity, etc). Please consult a local accountant, tax lawyer, and/or government official for advice.