Hi there! I am no expert on this, but thought I would share what I understand to be the case with outdoor grows. At least, this is my situation in southern Ontario, Canada. As of today, I will get 15 hours and 17 minutes of daylight. That increases by a few minutes each day, and will continue to do so until June 21, which is the longest day of the year, which will be 15 hours and 24 minutes long. After that, days will start getting shorter again, by a few minutes per day.
But my understanding is that the shortening days won't trigger flowering, but rather the number of hours of uninterrupted darkness. Around August 15 (where I live), darkness will hit 10 hours and increase (daylight of 14 hours and decreasing). Apparently that will trigger flowering. I will have about 8-10 weeks before first frost, from mid-August to mid/late October.
Not sure exactly where you are located, but imagine that you could find the corresponding daylength, darkness etc. data for your area. I do believe that June 21 will be the longest day of the year for you, as long as you are in the northern hemisphere.
Hope this helps a bit!