I can only get a price open for a stock on the latest trade day?
=GOOGLEFINANCE("BB","priceopen")
However, adding a date returns #NA.
=GOOGLEFINANCE("BB","priceopen", "2020/12/17")
every day the open price will close, so if you want to know the past value for some date use:
=GOOGLEFINANCE("BB", "close", "2020/12/17")