To change when calls are forwarded to your phone, go to Extensions, then click Edit on the extension you want to change. Then, select Edit next to the forwarding number you want to update. Then click the drop down for 'When do you want this number to receive calls?' to change the forwarding time:
You can choose from 24/7, Monday through Friday, 9-5, Saturday and Sunday, 9-5 or design you own custom schedule and choose the hours for each day you want to receive calls.