Parents 50th Wedding Anniversary

My sister and I have been planning my parents 50th wedding anniversay party, a relatively modest dinner party, for the better part of this year - to happen in December. We discussed the date and the fact that we would like to do something for them, without revealing details early in the year, to make sure they would not make plans for this day. There were no objections, and we kept the bulk of the plans secret as a surprise. Now, 60 days before the party, when invitation have been sent, airline reservations have been made and money expended for a deposit at the restaurant, my Mother expresses that she feels we are "hijacking her special day." The dinner party is not even on the actual day. I am at a loss and don`t know how to respond. Besides the fact that I will burn for approx. $1,500 at this point if we cancel, and that I will have to disinvite about 30 people, I am stunned that something my sister and I wanted to do to honor my parents is being characterized as a "hijacking." My Father expressed to my sister yesterday that he is concerned we can`t afford a party. But shouldn`t we, as adults, be trusted to know what we can afford? We can afford it. I havn`t spoken to them yet, after my sister called, crying, last night and left a phone message relating all this. I just don`t know how to handle this and don`t want to do anything to create a permanent rift in the family (other family members have made non-refundable travel plans, also). I certainly don`t want to force a party on my Mother, who I`m sure is the main force behind not wanting a party. My Father seems caught in the middle. Can someone give an outsiders viewpoint?