It's normally 5:00 p.m. but during the free dining promotion, they're opening it early to accommodate extra people.
We had one at 5:10 in July, and arrived a few minutes early. They were still not letting people inside yet, but they did open promptly at 5:00 and start seating people.
We have ADRs the first week in October at 4:30pm, which was supposedly right at opening. Four years ago, the first and only other time we ate at Boma (and loved it!!), they opened at 5pm.