The initial pressurisation, slightly above ambient pressure, begins when the aeroplane reaches 65 kts ground speed.
From and old 744 tech Manual
The cabin altitude controller automatically sets cabin altitude slightly below destination field elevation so that the cabin is pressurised slightly on landing. At touchdown, the outflow valves open, depressurising the cabin.
The maximum allowable pressurisation for takeoff or landing is 0.11 psi above ambient.