you need to go to
unrevoked, flash the S-ON tool.
Then go to
Shipped ROMs, and download the shipped rom of your choice (you say you want the one that was there on day 1, I am sure a newer one is a better choice though.)
After you download the shipped rom, you can run the RUU .exe on your PC. If you are on a mac, things may be a little more complicated for you, but are doable. (I am sure there are threads about doing it on a mac, options including but not limited to BootCamp, VMWare, Parallels, etc...)
NOTE: It is important to do it in that order, because once you flash the RUU, you will not have a custom recovery to install the S-ON tool with. If you run the RUU, then flash a custom recovery, you will no longer be like you were on day 1.