You can save permanent settings for the Day View using the options displayed in the User Preferences Window (see User Preferences | Organizer to read more about Organizer preferences). When the Day view displays for the first time, the Day view settings are defined by the User preferences. Thereafter, you change the settings using the buttons displayed in the top left of the view.

Day View Settings
1. Decrease Time interval button - reduces the Time interval for the Day. Valid Time Intervals are 15, 20, 30, and 60 minutes.
2. Increase Time interval button - increases the Time interval for the Day.
3. Decrease Number of Days button - selecting this will reduce the number of Day columns.
4. Increase Number of Days button - selecting this will increase the number of Day columns to a maximum of 7 Days.
5. Time hour Format button - select this to display all event times by 12 hours or 24 hours.
Further reading from Changing Day View settings: