Whitman Mission National Historic Site is located in Walla Walla, Washington, and holds significant historical and cultural value. Here is a summary of the site, including reasons to visit, specific points of interest, interesting facts, and the best time of year to visit, verified across multiple independent sources.

Reasons to Visit:
1. Historical Significance: Whitman Mission was established in 1836 by Dr. Marcus Whitman and his wife, Narcissa, as a Christian mission to the Cayuse Native Americans. The site played a crucial role in the Oregon Trail migration and the westward expansion of the United States.
2. Cultural Experience: Visitors can learn about the interaction between early settlers, missionaries, and Native American tribes, gaining insights into the challenges, conflicts, and impacts of westward expansion on the Native American communities.
3. Educational Opportunities: The site offers various educational programs, exhibits, and tours that delve into the history, lifestyle, and experiences of the Whitmans and the Cayuse people.

Points of Interest:
1. Whitman Mission Visitor Center: The visitor center provides an introduction to the history and significance of the site through exhibits and artifacts.
2. Historic Structures: Visitors can explore the reconstructed mission house and blacksmith shop, gaining a glimpse into the daily lives of the Whitmans and their mission work.
3. Memorial Monument: A memorial monument stands at the site of the tragic event known as the Whitman Massacre, honoring those who lost their lives and the complex history surrounding it.
4. Interpretive Trails: The park has interpretive trails that guide visitors through the historic landscape, showcasing the natural beauty of the area while providing historical context.

Interesting Facts:
1. The Whitman Mission was the site of the first organized wagon train to cross the Rocky Mountains, known as the "Great Migration of 1843," which played a crucial role in the settlement of the Oregon Territory.
2. The Whitman Massacre, which occurred at the mission in 1847, resulted in the deaths of Marcus and Narcissa Whitman, as well as 11 others, leading to tensions and conflicts between settlers and Native American tribes in the region.
3. Today, the site is not only a National Historic Site but also a National Historic Landmark, recognizing its exceptional historical significance.

Best Time to Visit:
The spring and fall seasons offer pleasant temperatures and fewer crowds, making them ideal times to visit the Whitman Mission National Historic Site. However, it is open year-round, and each season has its own unique charm. Summer provides opportunities for outdoor activities, while winter offers a serene atmosphere and occasional special events.

Sources:
1. National Park Service - Whitman Mission National Historic Site: https://www.nps.gov/whmi/index.htm
2. Washington State Parks - Whitman Mission State Historic Site: http://parks.state.wa.us/507/Whitman-Mission
3. Visit Walla Walla - Whitman Mission National Historic Site: https://www.visitwallawalla.com/whitman-mission-national-historic-site/