how was the climate that time?
As May 18 dawned, Mount St. Helens' activity did not show any change from the pattern of the preceding month. The rate of bulge movement, sulfur dioxide emission, and ground temperature readings did not reveal any changes indicating a catastrophic eruption
1980 eruption of Mount St. Helens
As May 18 dawned, Mount St. Helens' activity did not show any change from the pattern of the preceding month. The rate of bulge movement, sulfur dioxide emission, and ground temperature readings did not reveal any changes indicating a catastrophic eruption. 
what day of the week was may 25 2008 UNANSWERABLE what led to this eruption? It initiated as a series of phreatic blasts from the summit then escalated on May 18, 1980, as a major explosive eruption.
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1980 eruption of Mount St. Helens [SEP] Landslide and climactic phase
As May 18 dawned, Mount St. Helens' activity did not show any change from the pattern of the preceding month. The rate of bulge movement, sulfur dioxide emission, and ground temperature readings did not reveal any changes indicating a catastrophic eruption. USGS volcanologist David A. Johnston was on duty at an observation post approximately north of the volcano: as of 6:00 a.m., Johnston's measurements did not indicate any unusual activity. At 8:32 a.m., a magnitude 5.1 earthquake centered directly below the north slope triggered that part of the volcano to slide, approximately 7–20 seconds after the shock. The landslide, the largest in recorded history, traveled at and moved across Spirit Lake's west arm.