At the beginning of each end step, if a +1/+1 counter was put on a permanent under your control this turn, put a +1/+1 counter on this creature.
"What is that horrible flesh-noise?! We must silence it before it grows even louder!"
—Thxmir, Machine Legion scout
(20/09/2016)
Fairgrounds Trumpeter's ability triggers if, at any point during this turn, a +1/+1 counter was placed on a permanent that you controlled as the counter was placed. It doesn't matter whether you still control the permanent or whether it still has a counter.
(20/09/2016)
If a +1/+1 counter hasn't been placed yet at the moment an end step begins, Fairgrounds Trumpeter's ability doesn't trigger at all. If another ability triggers during the end step and puts a +1/+1 counter on a permanent you control, you won't put an additional +1/+1 counter on Fairgrounds Trumpeter.
