What is the most important day for Sikhs to pray?

Sikhs have no holidays nor do they have any holy place. Any day and any place is holy to remember God. Sikhs do celebrate the birthdays and other ceremonies of Sikh Gurus which they have as a part of religion but they don't have any particular day as being holy or any place being holy.

im improving the other persons answer and Sikhs do have a holy place of worship a gurdwara it mean the gurus door. A gurdwara is the holy place of a Sikh! :) ;)