You could use a union:

select * from C,A ;
where A.date = C.date ;
UNION ;
select * from C,B ;
where B.date = C.date

>Table A contains current working data
>Table B contains historical/archived data
>
>Table C is a running log, with dates going back into archived periods
>
>What syntax would be used to get a value from either Table A or B depending on the date value in table C?