You must meet your deadlines! US universities will not accept late applications unless they have ‘rolling admissions’, meaning they accept students on a first-come, first-served basis.
Most deadlines occur between January and February, although they can be as early as November and as late as March. You must send your university everything they ask for, all of it in perfect order (follow the instructions very carefully), well before the deadline.
When submitting your application electronically (eg via the Common App website), make sure you know the deadline for pushing 'send'. If you are sending by snail mail, the date posted counts for nothing; if you are close to a deadline, send your application by an express service that will require a signature on delivery.
For Regular applications, the good news of your acceptances will generally arrive in April.
However, you may have heard about...
Early Decision/Early Action
Students who have planned well in advance and are keen to attend a particular university may wish to consider some form of early application. There are now three options: Early Action, Early Decision I, and Early Decision 2. These options are not available at all universities, though, so check each school’s website to determine their policy.
Early Action is a non-binding plan that requires students to submit their applications in early autumn (usually around 1 November), in advance of the (generally) 1 January deadline for Regular applications. Notification of acceptance is made by January, but students have until 1 May to accept or reject the offer.
The advantage is you will know much earlier and can plan accordingly. The disadvantage is that candidates who apply this way tend to be much stronger and rejection is more likely than in the regular admission pool.
A student can often apply under Early Action to more than one university, but check with each school you are interested in; some may specify that, if you submit an Early Action application to them, then you cannot apply EA to other unis.
Early Decision 1 (ED1) follows the same timetable as Early Action, but is a binding plan. By applying this way, you have made a commitment, should you be accepted, to attend the university.
For this reason, you can only choose one university to apply as an Early Decision candidate.
Reneging on an offer of admission could mean that no other university will admit you, so make sure your heart is absolutely set on this college before you send off the forms.
Early Decision 2 (ED2) is similar – a single application and binding commitment to attend the university, if admitted – but the application deadline is later (usually 1 January, in line with Regular applications). Students are notified of the ED2 decision by mid-February. If the answer is positive, students must withdraw all other applications. This is important because, unlike ED1 when they haven’t submitted any others, students often forget or think they can wait to find out what happens. If they don’t withdraw other applications, they risk losing the ED2 offer and damaging their school’s reputation.