paper on the structure of DNA published in the journal Nature on 25 April 1953 with a DNA Day sale.
Until 26 April 2016 FTDNA has discounts on the tests detailed here.
Update: AncestryDNA is on sale in Canada until Tuesday at $40 off the regular price, $109 compared to $149 regular. In the US the usual price of $99 is reduced to $79 until the end of day Tuesday.
There's a Mother's Day sale at 23andMe - for existing customers only, until 8 May.