Look on the Bright Side Day is observed on December 21, 2017. It is a day to be optimistic: Even if today with the winter solstice is the shortest day of the year and it is often cloudy during this season, you should remain optimistic!
Optimism is a mental attitude or world view that interprets situations and events as being best (optimized), meaning that in some way for factors that may not be fully comprehended, the present moment is in an optimum state. The concept is typically extended to include the attitude of hope for future conditions unfolding as optimal as well.
A common idiom used to illustrate optimism versus pessimism is a glass with water at the halfway point, where the optimist is said to see the glass as half full, but the pessimist sees the glass as half empty. (With material from: Wikipedia)