It's important to understand that the endosymbiotic theory describes an ongoing process, not a specific event with a single date.

Here's a breakdown:

* The initial event: The theory proposes that eukaryotic cells (cells with a nucleus and other internal organelles) arose from the engulfment of prokaryotic cells (cells without a nucleus) by larger cells. This is thought to have happened billions of years ago, with the first eukaryotic cells appearing around 2.7 billion years ago.

* Ongoing evolution: Since that initial event, the symbiotic relationship between the engulfed prokaryotes and the host cells has continued to evolve and diversify. This process continues even today, with new adaptations and changes occurring in both the mitochondria and chloroplasts, which are believed to have originated from these ancient symbiotic events.

So, there's no single date for when the endosymbiotic theory "happened". Instead, it's a process of ongoing evolution that began billions of years ago and continues to shape the diversity of life on Earth.