Cause steam normally updates on tuesday and for PSN its thursday and yes its speculation but if bethesda is smart they would release dawnguard on those dates because alot of games is coming out on mid/late august and early september.
Actually August 7 is a Tuesday as well.
The main reason why I would say August 7 for PS3 is that they are still missing the 1.6 patch and with how poorly Skyrim was released on the PS3, I doubt Bethesda wants to make things any worse.
But, with PC, they already have the 1.6 patch and the only things that need to be done to the content is that the (this is assuming they are porting it from 360 to PC) .ddx files need to be converted to .dds, the .xma files need to be converted to .wav, and the .nif files need to be converted from the 360 version's of NIF to the PCs. All of this is simple as Microsoft already has the tools (and so do Sony).
Except I do think it was made on the PC as it would be dumb to make it for the 360 only to port the content to the PC.