Night before/early morning I can just about understand, but if you're in a 9-5 (or for arguments sake, you work largely Monday to Friday), then receiving an email at 11pm on a Saturday night is simply not acceptable, and a sign of really poor culture.
That email can be sent at 11pm on a Saturday, but not land in your inbox until the Monday morning, had the sender taken a bit of care about your welfare, by scheduling it to send at 9am on the Monday.
Clearly they don't give a shit about when you're sat thinking about work. Some people can ignore it until the Monday, however, a lot will see the email on a Saturday night as an emergency, and either reply to it straight away, or sit worrying about it until the Monday morning.