Yes - late April thru late May is the usual season for fresh, although there are a few online companies that sell them frozen (fiddleheads do freeze well).
The edible "fiddlehead" is actually the emerging shoot of the Ostrich Fern. So once spring passes & the ferns have unfolded, the season for culinary use is over.
The Ostrich Fern is the only variety considered safely edible, although even then it should never be consumed raw, only cooked. So if woods wandering, never assume any "fiddlehead" you see is edible - all ferns emerge that way, & many are deadly toxic.