Simultaneity is a relative concept.

QUARKY PHYSICS

This video discusses the relativity of simultaneity. It shows how two events can be simultaneous for one observer and not simultaneous for other observers. The video also discusses a thought experiment to prove the relativity of simultaneity.

Lecture 01 (S.T.R)
https://youtu.be/bmIT8bCd5_I

Lecture 02 (S.T.R)
https://youtu.be/wKhRMRJmnzc

Source