Winter Break I (December 19 through January 2): most of the campus offices are closed. Any student wishing to remain on campus during this time needs to apply for winter housing and be approved by the DSO. DSO provides support for meals/food via Crimson Cash grants.
Winter Break II (January 2 through January 16): students still need to apply and be approved for housing on campus during this period, but the threshold for approval is lower. At least one dining hall on campus is open, and any student approved for housing is eligible to eat there.
Wintersession starts on January 17 when all of the Houses and Yards open and any student can return to campus. Read more about the housing, dining and program options for that period here: https://wintersession.college.harvard.edu/