Non-ADHDers - especially Non-ADHD men: Please Help!

I'm thoroughly confused!

Ok here's the thing:

My husband's 43rd bday is tomorrow.  My 5 year old is very excited and has been planning to make a bday cake for weeks.  We had a get together last weekend with friends, one of whom also had a july bday, so DD and I made cupcakes and decorated them for THAT celebration.  Now tomorrow is the actual DAY of DH's bday and my DD wanted to have a 'family' party.  we have balloons, silly string, noise makers etc... a few days ago, DH informed me that we have to go to a potluck with people who are also fellowship recipients for this summer's research thing-a-ma-jigger.  I told him that DD had been planning a party, and asked if we should just bring the bday cake to the potluck.  He is shy, and easily embarrassed, so I assumed he'd say no, which he did:  "I dont' really know any of these people, so No." is what he told me.

We are on an extremely tight budget this summer, so I did NOT buy necessary items for cake baking for tomorrow because he'd veto'd the whole cake thing.  But then tonight as I was getting the kids ready for bed he announced that he was going to go to the store and asked, "ok, what do you want? cake mix? what kind?" Since he told me he didn't want a bday cake in front of these strangers,  I had changed the plan with my DD and we were going to instead make him a surprise pancake breakfast, but I needed maple syrup, which i'd planned to get tonight.  So I told him to just get maple syrup.  He looked at me like I was nuts, so I wrote out an explanation on a piece of paper so DD wouldn't hear and get upset that I gave away the surprise: 'I was going to do a breakfast bday thing since you didn't want the cake at the potluck - but if you want a cake at the potluck that's fine.'  He let out a bunch of exasperated sighs and said, "JUST TELL ME WHAT TO GET!!!"  I started to panic, because I'd thought i was perfectly clear, and was confused.  I just said, "I'm sorry.  I really, really don't understand.  I thought you didn't want a cake at the thing." and he said, "Well, they said to bring dessert." and I said, "Yes, and you told me (and the kids) that we'd bring a watermelon."  he replied, "well, we can bring a watermelon AND a cake.  It'll be fine."  All of this was in a very impatient, angry tone that I could see and hear but had NO clue what it meant or why he was so annoyed with me. 

finally I just told him to get a cake mix, and some vanilla frosting.  He then decided that it was too late to go to the store and he'd go in the morning.  I said, "But... if you are going in to work at 9, why do you need to go to the store?  I've got plenty of time to go tomorrow..."  He just got annoyed again.

What did I do wrong?  And what should I do tomorrow?!?  Should I do the cake or not?  I CAN'T do the pancake thing now because i've got no syrup.  I can't get out early enough to buy some and have everything ready by the time he's got to leave.  Why did he change his mind all of a sudden???  And if he did just change his mind, why didn't he just say that?