Alfred Nobel’s will (excerpt) (Paris, 27 November 1895) stipulates that the Peace Prize is to be awarded:

…to the person who shall have done the most or the best work for fraternity between nations, for the abolition or reduction of standing armies and for the holding and promotion of peace congresses.

The nomination processes start in September each year and nominations must be submitted before February 1 of the year in which the prize is awarded.