I have noticed ever since the calendar came out that announced start times for host games and events that the "server too busy" message tends to show up far more than it used to. It is probably just a lot of guests trying to log in at that time so the servers can't handle the traffic. This is why I am still not sure about the usefulness of the calendar for now. I think it might be better if they would just list the events but not necessarily the times.
Keep in mind that, when there is not an event going on, guests are not all trying to log in at the same minute. So even when there are 4,000 on, they are spaced out amongst rooms and inside games (like Haunted Mansion). With start times posted, you may have hundreds of guests trying to log in at the same instant which means the "ping" the server needs to get and send back to connect you can't keep up with the traffic.