Govia Thameslink Railway, which runs Southern, Gatwick Express, and Thameslink services, is urging passengers to check their journeys online ahead of time.
On 4 January, engineering work means buses will replace trains between Purley in London and East Grinstead in West Sussex, and between Oxted in Surrey and Uckfield in East Sussex.
Trains between London Victoria and Reigate will also only run between Redhill and Reigate, both in Surrey.
Lucy McAuliffe, Sussex route director for Network Rail, said: “The festive period gives us a rare chance to make the most of lower passenger numbers to carry out vital work on the railway.
“The areas of track and infrastructure being worked on this Christmas are normally some of the busiest sections of railway in the country, so we will make the most of this window of opportunity to work while the railway is closed, as we aim to give passengers safer and more reliable journeys.”