Even though the admissions were only sent out yesterday, everyone is talking about housing and there are people who are 100% set on their hall already. How fast do these rooms fill up and when do most people sign up by? Since I’m from OOS I don’t know as much about uiuc housing as much as other people do.
Rooms do not fill during the time of applications for them. You need to apply for housing by May 15. All who apply by then will be in the same position on May 15. Toward the end of May, you will be provided a day and time when you can go on line and choose the housing you want. The day and time is assigned to applicants in random order so if you apply in May for housing you have the same chance of being chosen to go early in the process to choose a room as someone who applies in February. For some additional info see http://www.housing.illinois.edu/apply-now/faq
However, if you are interested in private certified housing I would not wait since its first come, first served. Some students will go on the UIUC Class of 2020 Facebook page and try to find a room mate. People also discuss different dorms on the UIUC reddit page. Congrats and good luck!