Chotrul Düchen, the ‘Day of Miracles’ is a Tibetan holy day that commemorates one of the four great deeds of the Buddha. It occurs on the full moon (the fifteenth day) of the first Tibetan month. The first fifteen days of the Tibetan New Year celebrate the fifteen days in which, in order to increase the merit and the devotion of future disciples, Buddha Shakyamuni displayed a different miracle each day.  The Festival of Miracles is celebrated by Tibetans from the first to the fifteenth of the first lunar month, so it is the first “Düchen” or “great occasion” in the Tibetan calendar. These fifteen days are considered multiplying days in which the effect of any action is magnified millions of times and the fifteenth day is especially great. Chotrul Düchen, like Saga Dawa Düchen, and Lhabab Düchen is considered a ten million multiplier day, multiplying the effects of all positive and negative actions 10 million times! It is therefore considered very useful to engage in positive activities today!

To commemorate the occasion, Tibetans traditionally will make lamps of yak butter, called butter lamps, in the shapes of flowers, trees, birds, and other auspicious symbols.  Great displays of light are offered in the evening.  We will celebrate by gathering in the morning for a teaching from Khenpo Karten Rinpoche on the meaning of the holiday and reciting prayers.   Practitioners are encouraged to light candles in the evening to commemorate the Day of Miracles.

Learn more:

The Story of the 15 Days of Miracles

Rigpa Wiki-Brief Explanation of Chotrul Duchen

Description above drawn from:

Diamond Way Buddhism blog

Wikipedia–Chotrul Duchen