I only have the one evening

How can you? Even if a guy asks for a specific date, he may forget to cancel if his plans change. At best, it's a tentative request that far in advance.

I've gotten numerous requests that are months in advance only to never hear from the guy again. I'm sure most other providers have, too.

So, go ahead and contact her now. There is no reason not to. But don't think that alone "books" your time slot.

Send another confirmation once you've purchased your plane ticket. And then another a week prior. And then another one the day before the date unless she gives specific instructions on how she wants to confirm the date.

If you contact now, not only can you find out now if she's available, but it also gives you plenty of time to shore up local references if her screening process is rigid.