Sat Nam. Every day is a feast day, if one does not go hungry! If you are asking what are most important holy days, then the most important is Baisakhi, the day on which Guru Gobind Singh gave the first Amrit ceremony. Then there are all the birthdays of the ten Gurus, and Guru Gaddi Day, the day on which Siri Guru Granth Sahib was installed as our living Guru.