Tonight is the Nomination Ceremony. I would prefer that it is posted sometime in the afternoon or evening, however you have until around midnight to get it up.
Open the ceremony up by announcing yourself as Head of Household. Remind everyone you have the power to nominate two people for eviction. You may give a few opening statements as to how you came to your decision and why, without revealing anything of course.
Once that is out of the way, begin pulling the keys. You will pull all but two, your nominees. Once your nominees are made public, you can address them individually and give some reasons why they find themselves on the block. Once you do this, adjourn the ceremony.