Long nights or cold temperatures trigger Christmas cactus to bloom. In mid-October, begin leaving your Christmas cactus in complete darkness for 12 full hours each night. Otherwise, keep caring for it normally, allowing the soil to dry just a bit between regular waterings. In six to eight weeks, buds should begin to form.
Cool temperatures will also prompt Christmas cactus to bloom. Instead of giving the plant 12 hours of darkness each night, you could subject it to nighttime temperatures of 50 to 55˚F. This should begin in early November to trigger bloom in time for Christmas.