Most foreigners who've been in Poland at Christmas may agree with me on the following:
The holiday is centred on the Christmas Eve or "Wigilia" meal, traditionally consisting of up to 12 fish and non-meat dishes. These typically include beetroot soup with "ears" (ask your friends about this), a lot of carp and poppy seed cake -- and no alcohol, so it's better not to bring a bottle of wine if you've been invited over. The meal should start around 4.30 p.m., or once the first star is visible. It's preceded by a tradition generally called "the wafer": everyone gets a thin, Communion-style wafer and goes around wishing each guest a Merry Christmas, adding elaborate hopes for that person's future, and takes a small piece of the other person's wafer (and vice-versa; rather like a series of toasts). Prepare for a lot of cheek-kissing. Presents are exchanged after the meal. If you can stay up so late, consider going to a Midnight mass (or there may be one at 10 p.m.), though expect a long service in a standing-room-only sanctuary.
So obviously it's unlike the usual British or American holiday, with presents opened early Christmas morning, and the main family meal on Christmas Day. I have found Dec. 25 to be rather a Christmas afterthought in Poland, though both this day and Dec 26 are days off. People watch a lot of tv: the younger generation has grown up with "Grinch" and "Kevin" ("Home Alone") as a seasonal tradition. Boxing Day is not a sales extravaganza -- yet.
Your workplace may hold a pre-Christmas gathering which is also called "Wigilia"; so if someone calls on you to attend a "Christmas Eve" party don't worry, it's much earlier than Dec. 24. Like Wigilia, it may feature similar food and the above-described wafer exchange. As a rather quiet, midday meal, often without alcohol (anyway, people have to drive), it's much different from the British/American blowout office party -- and I, for one, prefer the Polish variant. You can exchange seasonal greetings and share a good meal with your office friends without wild socializing with people you don't really know, and worrying the next day how big a fool you made of yourself!
Christmas trees and other decorations tend to appear later, and stay up later; the tree may not come down in churches until Feb. 2, officially the end of the season. Thus, you can go to Nativity plays and hear Polish Christmas carols well into the month of January. Even if you're not especially religious, you may find the Polish carols beautiful pieces of music (as indeed are English carols). It's better if a competent choir presents them to you, however; too often have these hymns been massacred by determined warblers at the local church, singing at quarter throttle.