Christmas Bird Counts give birders the opportunity to participate in a long-running citizen science project. By late December, most migratory birds have settled into their winter ranges. CBCs provide observational data to assess where those ranges are, how big the winter populations are, and whether there are any changes over the years. Most counts have been running for a few decades; some have been running for much longer.
You can find a schedule for New Jersey's CBCs here. Similar schedules may be available for other states or regions. You can also find the times and locations for CBCs in other areas at the National Audubon Society site. This year, I will be participating in at least two counts in New Jersey.