Complex sentence: Especially high activity of Hessdalen lights took place in December 1981-summer 1984 when lights were observed 15-20 times per week, gathering numerous tourists staying there overnight to see the phenomenon.
Split into simpler sentences: Especially high activity of Hessdalen lights took place from December 1981 until the summer of 1984 when lights were observed 15 to 20 times per week. The frequency of the lights caused a gathering numerous tourists staying there overnight to see the phenomenon.