It depends on what appointment type it is, and if there is availability on the date you want.
new service > call activations
existing service (was previously working)> call faults
You can't pick your appointment date, but it should be straight forward to get another one a little further down the track. Unfortuantely only Optus can do this for you. Try Facebook Optus or back to LIVECHAT (perhaps in business hours?)