I'll summarise what's happening:
- purchase between today and 2nd May
- travel between 25 April and 27 May (but almost every TOC will have "barred dates" - and you will have to look on each TOC website for these. Greater Anglia are barred every weekend I think!)
- for almost every TOC, only Standard Class Advance is available in the Rail Sale. The exceptions are: LNER (also includes 1st Class Advance), Avanti (also includes Standard Premium), GWR (also includes 1st Class Advance), Thameslink / Great Northern (Off-Peak only as they don't have any Advance fares) and c2c (Off-Peak only as they don't have any Advance fares)
- Merseyrail, Heathrow Express, Caledonian Sleeper, and London Overground / Underground are not included. I don't tink Lumo are either. It looks like TfW are participating, but I am not sure about Scotrail.
- now here's the catch: in all but a handful of cases, the only Advance fares that are included are "TOC only" and not "TOC & Connections". So if your journey involves two different TOCs, there will be no Rail Sale fares. This also means that there will be on cross-London Rail Sale fares. So it will be splitting heaven for many journeys.