I'm going to push hard against this, possibly breaking the shakey peace we have right now but its worth it. Imagine your parents telling you about divorce then celebrating your bday 12 hours later. WTF. My Ex is sick.
Planning to tell D the day before her birthday sounds messed up, but remember--you do NOT control your STXBW. You can request that she not tell D that day, and she will agree to your request or not, but pushing "hard" and "breaking the shaky peace" doesn't change anything except maybe allow you to vent. There are healthier ways to vent. The flip side of that is she doesn't control you. If you don't want your D told the day before her birthday, tell her tomorrow, get it out in the open well in advance of her birthday.
May'19 - separation. | Dec'19, Oct'20, Jan'21 - painful breakups. | Jan'21 - freedom! "We the ones who play hard, we live hard, we love hard, we light up the dark." -- Kesha