Please can someone help

I have a table, of dates and references. What I want to be able to do is
input a date, and it then lookup to see what the refernce should be for this.

IE
Date Reference
10/01/2006 Period 1
16/02/2006 Period 2
19/03/2006 Period 3
25/04/2006 Period 4

So if I enter a date of 25/02/2006 it brings back PERIOD 2 as this date is
greater than Period 2 but less than Period 3.

Many Thanks in advance for your assistance.