I am contacting you to express my dissatisfaction and dismay at your decision to postpone the film screening of (name of film) to another date. Everything had been prepared in advance and the film was ready for broadcast. I cannot comprehend the reason for your action. I am requesting a reply with your justification for the delay.

I am contacting you to express my dissatisfaction in the decision to postpone the film screening for a later date. I was under the assumption everything was in order to broadcast the film as planned and agreed. However, now the decision was made to delay its showing. -- If you can go into more detail about this "film screening" there are other things you could add as well. Such as how long it's been delayed, weather it's just yourself or a group of people affected by the new date and weather it was delayed for any valid purpose. Also, if there was a number of film screenings postponed to a later date, including the name of the film would be valuable. ** Also I see a couple good other answers already as well, but I figured I would give my recommendation. Best of luck and have a wonderful day!

Yes, it all makes sense and seems sufficiently formal to me. The only part that is quite informal is “and now this happens.” If you want to be more formal, you could change that to “and so I do not understand the justification for the delay.” Also, it should be “to another date” not “for another date.”

I am writing to express my dissatisfaction with the decision to delay the film screening *to* another date.