What are the conditions for the base invasion to happen?

What are the conditions which must be met before the aliens invade your base? Can it happen again?

When I got raided by the aliens, this was my current story progress:

Date: July 8th. Difficulty: Normal. Alien Storyline: I raided the alien base some weeks ago and researched the hyperwave beacon, but I hadn't start building the hyperwave relay yet. Exalt Storyline: Two missions ago I completed the third covert ops extraction mission.

As far as I know, the invasion triggers 14-21 days after you assault the alien base. I can confirm you do not have to build or research the hyperwave beacon, as it triggered before I did.

Based on in-game dialogue, it should be a one-time event. I know it can occur as early as June on Classic, and does NOT require researching the Hyperwave Decoder. I believe but cannot yet prove it's triggered by the first event in your "Alien Storyline" in your spoiler - it happened to me only a few days after that event.

Edit: Ufopedia.org confirms it's a one-time event. Regarding timing: "Base Defense can take place after May and at least until September." It lists no other conditions, so either I'm wrong or they haven't noticed the dependency yet. I imagine I'm wrong; ufopedia's usually pretty solid.