Deadlines vary by REU site. Typically, most applications are due before March 1st.
NSF funds a large number of research opportunities for undergraduate students through its REU Sites program. Each REU Site consists of a group of ten or so undergraduates who work in the research programs of the host institution. Each student is associated with a specific research project, where they work closely with the faculty and other researchers. Students are granted stipends and, in many cases, assistance with housing and travel. Undergraduate students supported with NSF funds must be citizens or permanent residents of the United States or its possessions. An REU Site may be at either a US or foreign location.
The award varies by REU site. Typically, students receive a generous stipend (~$6000). Housing is also often provided as is transportation to and from the REU location.
The National Science Foundation website and Blackboard are your primary sources for this award. Undergraduates will gain access to the CUFR Blackboard Organization in their first consultation.