On the morning of the shooting, he already planned to not go to school since it was valentine's day and he told the people he was living with that "he doesn't go to school on valentine's day". That afternoon, he tries to call his ex-girlfriend who hung up after 9 seconds and told him to leave her alone. This is what probably set him off from there because, an hour or two later, he calls an uber who takes him to stoneman douglas and he then carried out his attack. So i don't think feb 14th was a date he always had in mind, it's just that the specific chain of events that happened that day led him to bring to life what he always wanted to do. (Also, the night before, he claimed he heard an evil voice in his head telling him to burn, destroy, kill).
"I need a change of scenery, I think we both do … We can all use a good cleansing of mind and soul from time to time!"
- a text from Nancy Lanza, 4 days before her son committed the Sandy Hook Elementary School shooting