The UK and Ireland offer reduces the price from £99 to £79 and expires at 23:59hrs on Monday. These prices don't include shipping costs or applicable taxes, which are added to the bill.
The test maps your ethnic mix going back multiple generations and can also help identify relationships with unknown relatives through a dynamic list of DNA matches.
According to Ancestry's corporate statement yesterday, there are already 850,000 people in the AncestryDNA database, so you have a very real chance of connecting with a distant relative. And such relatives may well hold information vital to your family research!
The same promotion is also being run by AncestryDNA.com in the US. This is available only to those in the USA, and costs $79+shipping etc. It will end at 23:59 ET on Monday 27April.
By the way, Saturday 25 April is DNA Day because it commemorates the day in 1953 when James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin and colleagues first published papers about the structure of DNA. It's been recognised since 2003.