PhONEday was a change to telephone numbering in the United Kingdom on Sunday 16 April 1995. A shortage of unique telephone numbers in the old dialling system meant that it was becoming increasingly difficult in certain areas of the country to assign unique numbers to new subscribers. To counteract this, dialling codes starting with 0 were changed to start with 01. In Bristol, Leeds, Leicester, Nottingham and Sheffield, the existing area codes, were entirely replaced with new codes comprising 4 digits, and the subscriber numbers lengthened from 6 to 7 digits. The changes made possible the ability to provision new numbers in each of these five cities. It also had the effect of assigning all geographic landline telephone numbers into one range of numbers starting with 01, allowing for further changes to be made at the Big Number Change in 2000. A £16m advertising campaign, and an eight-month parallel period during which both old and new codes were active, preceded the change. PhONEday followed a change made in May 1990, when the old London area code 01 was released from use, permitting all United Kingdom geographic numbers to begin with this prefix. Originally planned in 1991 to take place in 1994, in 1992 the change was postponed until the Easter Sunday bank holiday in 1995.