Greece here! We put on decorations pretty much when we please, as far as it’s before the Christmas eve. The timeline is:

• December 6th, St. Nicholas day: rather than gift giving, this day is attributed to sailors. There are special ceremonies held by the coast by decorated boats
• December 24th, Christmas eve: Kids go door- to-door and sing christmas carols (κάλαντα, /kalanda/) accompanied with triangles and drums. It’s a cacophony, but it’s cute. They usually expect money or sweets in return (money is more appreciated) or even both, so make sure you’re stocked with coins! Some religious households bake “christopsomo” (χριστόψωμο, meaning Christ bread), a ceremonial round loaf of bread with religious markings on top. There’s a midnight mass for the religious ones too.
• December 25th: still no gift giving, this day focuses on feasting and gathering up, usually famillialy. Tables overflow with festive cooking, including christopsomo, melomakarona (μελομακάρονα, a syrupy biscuit) and kourampiedes (κουραμπιέδες, a spicy, dry, crumbly biscuit with a generous sprinkle of icing sugar, enough to make it look snowy). Some of the most religious have been fasting for quite some time till this day comes as well.
• January 1st, new year’s, st. Basil’s day: Now’s the time for gifts! St. Basil (Άγιος Βασίλης, Ayios Vasilis) is our own version of Santa Claus, so the children get gifts on new year’s instead of Christmas. Another feast happens, and usually ends gloriously with the Vasillopita (Βασιλόπιτα) which is something like a dry cake with a single coin inside. It’s split radially in pieces or in squares, each one given to a single member of the table, except for some that are “given” to Christ, to the holy Mary, to st. Basil, to the house, etc. so you always cut more pieces than you actually need. Someone’s piece will have the coin inside, they “win” the year and are said to have good luck. Companies also tend to hold Vasillopita ceremonies later in the year, and in those ones whoever wins the coin gets generous gifts, like a bonus or extra time off.
• January 6th, Epiphany (Θεοφάνεια, Theofaneia): The Christmas season concludes with Epiphany celebrations, where brave souls dive into cold waters to retrieve crosses blessed by priests, commemorating Christ’s baptism and marking the end of the festive period.