I wouldn't want to just leave it open to “as early as convenient” because convenience can be defined in different ways. I think “any time up to April 9, including April 9” would be good, but I'm okay with just saying “April 9”. It's simple. Let's just go ahead and get her here.
If there is an issue.... Maybe the clerk could inform us as to what would happen if Ms. Tait says she is unavailable then.