During the Christmas season, when my daughter was 2 years old, it was very hard for her to understand getting presents under the tree. She just wanted more and more, didn't take time to see the gifts she had already received, and could not wait while other people opened presents. The GET GET GET was just too much and resulted in screaming, demanding and a stressful tree time experience.
In order to better prepare for next year, we played "Christmas" a lot with her toys who would take turns coming to the tree and receiving a gift. She would help the toy pretend to unwrap the gift and then shout loudly what it was. I would then ask lots of questions about the toy and make comments. This helped her to remember during the holiday season that when it was time to unwrap the presents, everyone takes turns and we appreciate each gift before moving on.
We cannot expect little kids to understand the niceties and the traditions of holidays when those activities are so different from the rest of their lives. We suddenly bring a tree into the house, they are offered treats at every turn and there is unfettered acquisition of new stuff. Some practice and lots of pre-discussion is key.
Now that my daughter is 7, we have a clear expectation of the number of gifts and that really helps as well. She knows what the extent of the gift giving will be and knows that the last present is really the last present. We also now have 2-year-old twins who so loved trick-or-treating this year and stood asking at each door for, "more" and "bigger one," that we're realizing we'll have to practice this tradition with them, too!