First I would like to thank everyone for their comments.
I'd like to say that although DH's aunt and uncle are not my favorite people, I'm not blaming them for the last minute anniversary party.
It was a BBQ in the back yard sort of thing, so not fancy not that that matters.
No, I have not told or made my kids aware of my irritation with my inlaws in regards to parties, and in general I still very much love my MIL. I just know what priorities she puts on things. Her and FIL (who passed away in 2012) have done things like this before. Examples:
1) FIL wanted to hang out with DH, me and the kids one Sat, and invited us to their house (which is an hour away). That morning, we got in the car and were about 20 min down the road and DH called to say we were on the way. MIL said that FIL had gone to visit a friend instead, didn't know when he'd be back, and that she was also unavailable. We had to turn around and go home. HUGE disappointment for the kids.
2) IL's again invited us over for the day, and mentioned they needed help with the computer down at their shop (family business). We went down, FIL spent an hour washing his motorcycle not really talking to anyone, and MIL during that time was working on her online class in the computer room. We went down to the shop and DH (who is an IT guy) fixed their computer, and we thought we were all going back to their house, only at that moment the ILs said they were going to a friend's house for the rest of the day. We drove home.
3) Last summer, MIL wanted to take a family vacation with herself, her 3 sons and their families as a way to remember FIL who had passed the previous year. MIL paid for the rental houses but asked me to plan horseback riding, a guided fishing trip on the lake and other things. MIL flaked out on the horseback riding last minute and sat in the boat and didn't fish on the guided trip either. For most of the trip she stayed in her rental house. This was not so bad, but just shows how she changes her mind last minute on certain things. I would not have planned horseback riding had she not asked me to.
Also, I'm not at all saying I don't want or that my kids shouldn't have a relationship with my mother in law. I'm only saying I'm not going to waste time planning formal events in advance for people who change their mind at the drop of a hat. A casual dinner or lunch or us visiting MIL (she rarely comes to our house) will suffice at least when it comes to kids birthdays. So when she asked about a party, that's why I said we were planning birthdays in a different way.