There is no universal answer to this. For starters: Not everyone does celebrate Christmas.
From my personal observations it would appear people celebrate Christmas simply out of tradition (There has always been a Christmas to them).
Do NOT get me wrong, I know of people who celebrate it because of its Religious implications.
I also know people who celebrate it simply because: "Why not?"
As I stated, there is no universal answer.
People celebrate functions for many reasons. A special marker in a lifetime, such as a birth, a death, or a wedding are all causes for a celebration. While a funeral is not a joyous celebration, it is still a celebration to remember a major event in life.
Religious celebrations are frequently mandated by that religion. Once again, it is a major milestone within the religion. Such events as the victory of the religion's leader over adversity, a personal communication by the Deity of that religion to a spiritual leader, or the founding of the religion are all causes for a remembrance. These remembrances are called celebration.
Remembering and celebrating are man's way of passing on events to future generations.
families mostly celebrate Christmas in their homes
They celebrate Thanksgiving with their families over a computer or with phone calls if they are unable to celebrate it together in person.
yes they do celebrate harvest
They celebrate at home together with their families
not necessarily. Different families celebrate it differently; some it's a big thing others it's not. Some families don't celebrate because of their religious differences.
Mothers and their families.
Families celebrate thanksgiving just out of the tradition that has lasted many years.
Both families got together to celebrate the new year!
families celebrate by gathering and going to church, and then having a family meal.
To mark time and stages of life.
They celebrate mihihi
usually