Is it a good idea to keep Request querystring as a property?
Can you guarantee that eventId will never change? If so it might not be a bad idea, but this comes down to a few things


eventId never changes;
how and where you're using it. It might be somewhat difficult for a new programmer in the project to locate and realize what it actually does (it returns the eventId for the current Http Request), solely based on the property name; and
Are you sure that both HttpContext.Current and Current.Request are initialized before retrieving eventId. There is a change these will throw a null reference exception. It might be a very good idea to do a simple if != null check before invoking the Request property.


If your code can accommodate for these points I don't see any inherently wrong with doing this.