You will need to contact AirBnB and hope they understand that you did not coordinate your calendars and deem it an appropriate reason to cancel on a guest. If you are new to hosting and it's your first time then you may be lucky but they are not that keen on hosts who don;t maintain calendars!
@Anna226, I have done this once too, by mistake. Rang Air and they would not change it. I was new at the time and I think this happens to more than one host. Someone posted the other day that it would be nice to have a 5 minute ''regret, oops what have I done now'' window but there isn't and you might just have to wear it.
Of course, you could try Air but I doubt they have softened in that regard.
review and if I cancel it will make me look like a flaky just which I'm not at all. There used to be a phone number to click on and now it's gone, and there no email to reach Airbnb. I can't believe it !!! What are we going to do ? Ed