The Fifteen Days of Miracles—from the first day of the Tibetan new year (Losar, February 28) until the fifteenth (March 14)—commemorate the special time when Guru Shakyamuni Buddha showed miraculous powers in order to subdue the opposing tirthikas (Six Founders), who lacked faith in him, and to inspire more faith in his followers. He performed a miracle on each of the 15 days which culminates on the full moon, the fifteenth day of the lunar calendar, which is called Chotrul Duchen.
All fifteen days are merit-multiplying days, when the merit of virtuous actions performed on these days is multiplied by 100 million, as cited by Lama Zopa Rinpoche from the vinaya text Treasure of Quotations and Logic.