Need to replace a value in a SQL update statement based on looking for valid days in a table

I'm looking at a SQL statement where I need to access the prior day's data in a table that has a unique index by symbol + trade_date.  I can do something like this:

update fdd1
    set fdd1.Prior_Open = fdd2.Day_Open,
          fdd1.Prior_Close = fdd2.Day_Close
    from full_daily_data fdd1
    inner join full_daily_data fdd2
    on fdd2.Trade_Date = DATE_SUB(fdd1.Trade_Date, INTERVAL 1 DAY)

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but that won't take into account holidays and weekends.
So, if I make a single column VALID_TRADING_DAYS table that contains just the days that are valid for the trading data I have, how could I get that info out of the table and replace it into the "1" in the statement above?

Psuedocode-wise, this is how I envision it:

Days_Back = 1
Select Trade_Date as prior_trading_day from VALID_TRADING_DAYS where trade_date = DATE_SUB(fdd1.Trade_Date, INTERVAL Days_Back DAY)
while no rows returned
   Days_Back = Days_Back +1
   Select Trade_Date as prior_trading_day from VALID_TRADING_DAYS where trade_date = DATE_SUB(fdd1.Trade_Date, INTERVAL Days_Back DAY)
wend

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Then I could have this SQL statement instead
update fdd1
    set fdd1.Prior_Open = fdd2.Day_Open,
          fdd1.Prior_Close = fdd2.Day_Close
    from full_daily_data fdd1
    inner join full_daily_data fdd2
    on fdd2.Trade_Date = DATE_SUB(fdd1.Trade_Date, INTERVAL Days_Back DAY)

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But what would be the proper syntax for that in one SQL statement?