Let me clear something up first. You MUST keep your indoor schedule the same as your outdoor sunlight. So if your going to put a clone out june 20th, your indoor light schedule should be on at 5am, off at 9. (check our local daylight hours, add 1/2 hour on both sides for sun up and sun down) If you do all this, then your plants wont flower.
If your on a 18/6 or worse, a 24/0 schedule, and you put your plants outside, your plants will flower every time.
Seeds can be put out earlier, and I try to plant on holy week. That is in April, and that is due to moon cycles.
June 20th is the longest day of the year, that is why clones are safest on that date.