Setup instructions below, once you set it up once, it will save and automatically connect every time.
When you connect to a Wi-Fi network, your BlackBerry device saves the network information. The next time you're within range of the saved Wi-Fi network, if Wi-Fi is turned on, your device connects automatically.
On the home screen, swipe down from the top of the screen.
If the Wi-Fi switch is Off, set it to On.
Tap a network.
If the Wi-Fi Protected Setup screen appears, do one of the following:
If you have access to the Wi-Fi router and it uses Wi-Fi Protected Setup, tap Press WPS Button on Router or Enter PIN into router. Complete the instructions on the device screen.
If you don't have access to the Wi-Fi router or it doesn't use Wi-Fi Protected Setup, tap Don't use WPS.
If necessary, enter the username or password for the Wi-Fi network.
I thought you were looking for a feature where the wifi is enabled at a certain of the day (ie when at home). Some people do this manually to maximize battery life. On some phones it uses the GPS to determine the phone location.
Scanning for known Wifi networks waste battery life.