8-Day Cuckoo Clocks are fitted with a mechanical movement that requires winding every 8 days. At the full hour a little door opens to reveal the cuckoo bird which calls the number of hours. The cuckoo bird also calls once on the half-hour. The difference between a 1-Day and a 8-Day clock is mainly the size of the weights. The 8-Day clock weights are bigger therefore more heavy than the 1 day weights.