Why? Why not just make the announcement rather than tell everyone you are going to make an announcement and what will be in that announcement. The announcement is pointless, we already know what you're going to say.
No-one in real life says "Tomorrow I will announce that I'm pregnant or leaving you or buying a puppy." So why do politicians do it?
I'm not BU, am I?! If I am then tell me that you'll tell me I am tomorrow.