In Eve's vision of the clock seen as a countdown from the ETs' perspective (small blue beings at the other side of the wall where this clock appeared), the fewer opened lids of the advent calendar could be the 4 remaining days before Christmas.


Understand that all those explanations aim at showing the readers that the ET intel, even if cryptic, are meaningful, implying that they are trustworthy. This is key for the future chosen ones to remain faithful when receiving such telepathic and visionary intel themselves. This could be weird, cryptic but reliable.