The typical consultant answer is 'test it' but there are some good places to start.

People generally start checking email when they show up to work (yes that includes personal). Sending around 8:30AM will give you a good shot of being at the top of their inbox then.

Another good time is before lunch break. Shoot for something around 12:00PM for this.

If you have an email provider that allows you to adapt your send to the local time, use that. But if not, go for EST. Close to 50% of the US population lives in that, with another 30% one hour behind in central.