Mail Signing Certificate – What are the impacts when revocated?

Suppose I open a signed mail on 1st January 2000, signature is OK. What about this two cases :

1) If next CRL is to be published one week later, I found it strange that it is “probably” valid until next CRL, this mean that I should wait one week to “trust” the signature ?

2) I re-opened it on 31st January 2000, the signature is marked as invalid/revocated. How can I know from when exactly it became invalid ?