MIL taking son on 2nd birthday... Not cool lady


I know I just need to talk to her but I'm not sure what to say. My MIL watches my son on Wednesdays so I can work from home without distractions (I have no set hours). When she realized his birthday fell on a the same day she said "well I'm taking him the whole day (which never happens) and I guess if you want you can come over for cake and presents". Ummm.. no.

At the time I didn't say anything because I honestly didn't even really take it in. What should I say back? I'm taking MY son for the day.