HOH Nominations are held once a week on Friday roughly between 2 and 9pm. A typical Nomination day morning starts with the Head of Household meeting every house guest and letting them “plead” there case in the hopes of not getting nominated. The HOH has to then choose 2 people to put up “on the block” with the risk of one of them going home. Nominations are not done on the feeds but once the Live Feeds are back you know the results of the nominations usually within the first 15 minutes.
The Nomination Ceremony is conducted with the HOH placing all but 2 peoples Keys into the keyholes. Each key has a house guest name written on it, the name cannot be visible until the Key is removed from the hole. One house guest pulls a key out and passes the lock to whoever name they read out. It is now that house guests turn to pull a key out and pass it along. If your name is called you are not nominated. Here is a clip from youtube of the first Nominations ceremony on Big Brother 10 http://youtu.be/WBce49E7hhk?t=38m44s