Maintaining a greenhouse takes a lot of work and this increases during the colder months as monitoring the internal environment becomes even more important.
Your greenhouses internal environment should be kept the same all year round, so some extra input may be required maintain an equilibrium. You must monitor and record the internal temperature of your greenhouse each day at set intervals to make sure you have a true idea of what you need to change. If your greenhouse is too cold, plant will begin to die. So you may need to add a heater to keep the temperature up. That said, ventilation always remains important, but opening vents may cause a further drop in temperature, so make sure your vents are open as small as possible to provide adequate air circulation but not to alter the temperature.
You may find an increase in slugs and snails trying to enter your greenhouse during these cold months, so make sure you have repellent to keep them at bay.