Today may be Thanksgiving for some, but it's also the National Day of Mourning for others — here's why

For many Americans, Thanksgiving is a time to celebrate the fall harvest. The tradition is rich in history and dates back to 1621, when English colonists in Plymouth shared hearty meals with the Wampanoag tribe, a community of Algonquin-speaking Indians who once inhabited parts of Rhode Island and Massachusetts.

More than 200 years later, in 1863, President Lincoln finally declared the festive event an official holiday. Since then, it has been the hallmark of American celebrations.

Still, not everyone celebrates Thanksgiving — and for understandable reasons.

In 1970, Native Americans first came together on Cole's Hill in Plymouth to instead observe the National Day of Mourning — an annual reminder of the "genocide of millions of Native people, the theft of Native lands, and the relentless assault on Native culture," according to the United American Indians of New England (UAINE).

Over the years, the community has buried Plymouth Rock, the site where William Bradford, the founder of Plymouth Colony settlement first arrived in 1620, arrived. It has also placed a Ku Klux Klax sheet over a statue of Bradford.

"We are mourning our ancestors and the genocide of our peoples and the theft of our lands," the statement on UAINE's website reads. "It is a day when we mourn, but we also feel our strength in political action."

The "genocide" here refers to the number of massacres that followed the European colonization of the Americas. Perhaps the most notable is the Pequot Massacre, which took place in 1637.

In the spring of that year, the Pequots, a tribe in Connecticut, grew increasingly frustrated with Puritans who had spread from Massachusetts Bay into southeastern Connecticut. Conflict ensued, leading to the deaths of 13 colonists and traders. When the colonists began to mobilize in an effort to punish the Indians, nearly 200 Pequot Indians banded together and attacked a settlement. In the process, they killed six men and three women.

In May, the Puritans and their Indian allies responded to the Pequot attacks by carrying out three separate massacres in a number of Pequot villages. Only a small number of Pequot Indians survived, and those who did were either sold into slavery or joined other tribes. A day after the last massacre occurred, John Endecott, the governor of the Massachusetts Bay colony, reportedly celebrated the Puritans's victory by organizing a feast.

The Pequot Massacre, however, was just one of many incidents in which Native Americans were slaughtered by the masses. In the aftermath of America's "founding" in 1776, multiple massacres continued to take place.

Today, historians believe the Bear River Massacre of 1863, which took place near what is now the Utah-Idaho boundary line, is the deadliest attack on Native Americans by the U.S. military. An estimated 250 Shoshoni Indians were killed that year.

The Native American community remains one of the most marginalized demographics in the U.S. today. Most American Indian communities live near the poverty line, with some tribes reporting unemployment as high as 85 percent, according to nonprofit organization Running Strong for American Indian Youth.

Even worse, more than 120,000 tribal homes lacked access to basic water sanitation just eight years ago, the Environmental Protection Agency notes. Because of these issues, Thanksgiving holds less significance for many Native Americans than it does for other racial groups.

"To me, Thanksgiving is a reminder of our resistance as Indigenous People navigating this settler society that continuously tries to erase and destroy us, yet we are still here," Allen Salway, a community organizer from the Navajo Nation, wrote for Paper Magazine last year. "I will spend it honoring my ancestors and their fight for survival."