What does it say about the culture of a company, when an executive assistant is sending emails on a sunday morning? We are in the same time zone and i sent the email on friday. I would not have expected and answer until say wednesday...
It doesn't say anything about the culture of the company. It says exactly what it looks like. Executive assistant replied on Sunday. Nothing more or nothing less. Do not over-read the situation.
Also, unless there is a reason you did not mention in the post, it does not make sense why you were not expecting a reply before Wednesday. Replying to emails is one the jobs EAs have. Whether they chose to reply on their holiday or during the workday is their business.
It simply says that you check your mailbox on Sunday.
Executives in most of the companies are not bound to Monday-Friday 9-17 work schedule, so I think it is normal for them to send mail whenever they feel like doing it.
It's up to you to set the boundaries on how you deal with those mails. You don't mention the executive ever pointed out a delayed answer to weekend mail, so it's not a big deal.