Hello how do you know when the special events are going to start??

No, special events are not announced in advance (at least no more than a day or so before you can see a notice in the game itself).

Just follow the Need for Speed: No Limits official twitter page for heads up. The special events are also announced inside game, usually 3-7 days before the event. Sometimes the announcement appears as a popup ad in between races, or you can click the Special Events button on main screen and it will show you a countdown to start.