The best and most rewarding time comes with a cloudless day of clear air offering spectacular views of the Lassen Volcanic National Park landscape. During the winter season access roads may be closed. Strong winds and avalanche danger will keep you away. Even summer days can be a challenge for those not prepared. I remember Labor Day of the year 2013, when Lassen Peak "disappeared" in the clouds most of the day. On such a day, you may prefer to explore some other park sites such as Bumpass Hell.
But now for those sunny-summit days: Lassen Peak Trail offers Lassen Volcanic visitors a great outdoor experience with the opportunity to climb to the top of a landmark volcano in northern California. The well-maintained, 2.5-mile-long trail is steep and rocky, ascending 2,000 feet from the parking lot at Lassen Park Road's high point (8,512 ft, 2594 m) into the thinner air around the summit (10,457 feet, 3187 m). On your hike, you will see short-cut trails that thoughtless hikers blazed up the steep slope off the first switchback. The list of Know before you go recommendations posted at the trailhead reminds hikers to stay on the trail, since short-cutting scars the landscape and takes years to heal. The trailhead panel also summarizes Lassen Peak's history:
"The five-mile round trip hike to Lassen Peak introduces you to the volcanic event that spoke to a nation. On May 30, 1914, Lassen Peak awoke from a 27,000-year-long slumber, blasting steam out of a newly formed summit vent. By the following May, some 180 steam explosions had left a wide crater in the mountain's top. At the time, Lassen was the only actively erupting volcano in the U.S., and the nation looked on with wonder. The climatic eruptions of May 19 and 22, 1915, swept clean forests, pastures, and homesteads in the valleys below Lassen Peak's northeast flank. And on August 9, 1916, Congress duly recognized the forces of nature by establishing Lassen Volcanic National Park for all time."
More detailed descriptions of Lassen Peak's past eruptions can be found elsewhere [1-3]. A series of small earthquake swarms beneath the peak's southwest flank occurred in 2009 . Lassen's volcanic activity is periodically monitored by the U.S. Geological Survey (USGS) measuring ground deformation and volcanic gas emission . You probably did visit nearby geothermally active areas such as Bumpass Hell and already have an idea of the ground and subterranean structures you are walking on. As long as the Volcanic Alert Level is normal and the Aviation Color Code is green, park officials will leave the Lassen Peak Trail open. It is your responsibility to check weather conditions and decide whether you feel safe to go.
When climbing along the summit ridge you will notice that there are two summits. After reaching the false summit, scale the real one and enjoy the magnificent view :
To reach the actual summit, climb down into the saddle, cross the snowfield, and ascend the talus on the northern rim of the crater. The views of Mount Shasta to the northwest, and the Devastated Area on the northwest-facing slopes [what about the northeast-facing slopes?] of Lassen, are best from here. The white cone of the radio tower is the single sign of human influence on the summit; the lookout pictured on the interpretive sign is long gone. It's you, the wind, and the views, and it is wonderful.
References and more to explore
 USGS Volcano Information: Lassen Volcanic Center [http://volcanoes.usgs.gov/about/volcanoes/cascades/lassenvolcaniccenter.php].
 National Park Service: The Eruption of Lassen Peak [www.nps.gov/lavo/naturescience/eruption_lassen_peak.htm].
 Malin Space Science Systems: The May 1915 Eruptions of Lassen Peak, California, I: Characteristics of Events Occuring May 19 [www.msss.com/earth/lassen/lassen2.html].
 Tracy Salcedo-Chourré: Hiking Lassen Volcanic Park. Globe Pequot Press, Guilford, Connecticut, 2001; pp. 42-46.