When the Lunar Module (LM) and Command Module (CM) were in lunar orbit, they were indeed in a state of weightlessness due to the effect of being in freefall around the moon. However, it's important to note that they were still under the influence of the moon's gravitational pull throughout their orbit.
The moment at which the LM reached the moon's gravitational pull occurred shortly after the two modules were separated. The LM was designed to perform a descent from lunar orbit to the moon's surface, and for this descent, it needed to initiate a powered burn known as the Lunar Orbit Insertion (LOI) burn. This burn was conducted by firing the LM's descent engine to slow down the spacecraft and allow it to be captured by the moon's gravity.
The LOI burn was performed at a predetermined point in the lunar orbit that was calculated based on the mission trajectory and timing. By executing the LOI burn at the calculated time, the LM would adjust its velocity and trajectory to enter into a lunar orbit around the moon. This burn essentially marked the point where the LM transitioned from a freefall state to being captured by the moon's gravitational pull.
The precise timing and calculations for the LOI burn were determined before the mission, taking into account factors such as the desired orbit and the spacecraft's velocity. Mission control teams on Earth closely monitored the spacecraft's trajectory and made any necessary adjustments to ensure the LOI burn occurred at the planned moment.
So, while the LM and CM were weightless in lunar orbit, the moment of the LM's capture by the moon's gravitational pull was planned and executed through the LOI burn, and mission controllers were aware of this moment based on the predetermined trajectory and timing of the burn.