How do I respond to the txt from my besties mom SUPPPER polietly?

So I got a txt this morning from my friends mom. Shes a little crazy and doesnt like me but she doesnt like almost all of my besties friends. the txt said "Bonjour Rachel, can you come Wednesday 20th instead of Monday 18th(tomarrow) @ 4:30? Merci!"(thier familys french). I have no plans to go over there on monday or any other day so I have no idea what shes talking about. How do I reply to this asking what she meant/ why I was supposed to come super polietly so I dont sound rude or make her sound stupid. Thanks! :)