Is there any real reason to worry about June 15?

Those details do not add up to evidence of a planned attack. A date, a moon phase, and an anniversary can be linked after the fact, but the claim would need specific, independently verified information about a threat or a plausible way it would happen. Without that, it is speculation.

I’d avoid passing the prediction along as though it were a forecast. Claims like this can make ordinary events feel ominous without offering anything testable. If it is making you uneasy, stick to practical updates about the event and travel, and give the speculation a rest unless credible new information appears.

A useful check is to ask what would count as evidence against the claim, and whether the person making it has provided anything that could be checked in advance. Vague references to symbolism can fit almost any date, so they do not reliably show that something will happen.