For flights from Helsinki to Lapland (e.g. Rovaniemi) use FinnAir's "Flexible +/- days" fare calendar search. Depending on day of the week fare difference can be substantial. https://www.finnair.com/us/gb/
BTW Rovaniemi is just south of the arctic circle and mid December is close to winter equinox - 2 plus hours of daylight and the daylight could be cloud obscured. https://www.timeanddate.com/sun/finland/rovaniemi?month=12&year=2017
If you want to meet Father Christmas, the Rovaniemi tourist attraction is Santa Claus Village: http://www.visitrovaniemi.fi/attraction/santa-claus-village/
We visited Lapland a number of years ago much further north flying in to Ivalo, northern most airport served by FinnAir. Great experience skiing amongst the reindeer but not for most tourist. Sami hunting village can be pretty rudimentary in amenities. The holiday lodges mostly cater to Finns and tourist looking for something exotic such as ice hotels.
A small town by most standards, Ivalo (Sámi: Avvil) is a metropolis in
these latitudes. With plenty of services and an airport busy with
Christmas charters, it’s a useful place, though with few attractions.
However, Inari’s Sámi culture and Saariselkä’s plentiful activities
are close by.
There was bus service from Ivalo Airport to Saariselkä. The Urho Kekkonen National Park is adjacent and considered the true home of Santa Claus: http://www.lifeinlapland.com/articles/lapland-travel-tips/real-home-santa-korvatunturi.html
Urho Kekkonen National Park is located along the Russian border in the
municipalities of Inari, Sodankylä and Savukoski. The park includes
Korvatunturi, where Father Christmas lives – at least according to the
The western border of the park is by national road 4, i.e. E75
(Kiilopää)–Saariselkä–Ivalo), with several coaches daily, including
coaches from Ivalo to Saariselkä and Kiilopää and coaches from
Nordkapp, Karasjok, Vadsø and Tana bru to Rovaniemi. You could come
e.g. by plane to Ivalo or Rovaniemi or by train to Rovaniemi and
continue by coach
We started and ended our Lapland ski tour at the Kiilopää Fell Center just outside the National Park. The Fell Center is served by coach from the Ivalo Airport.