If I have a date and want to lookup a value from a table. How can I do
that? -- Over the last 5 years a certain % has changed and depending
to the date I want the correct % connected to the right date.

Kind of hard to understand exactly what you are looking for. If you
have a table with 2 fields (i.e. a Date and Value field), then you can
simply query on it.

For instance:

SELECT [<value field>] FROM <table name> WHERE [<date field>] = #<date
value>#

This would return all the records matching that date and give you the
value field entries.

You must have a match though. In the simplest case here you must have
a date and value entered in each record.