Alright, that might not always be true. If you grew up on a farm, you might have very affectionate memories of haystacks. Nonetheless, the only correct spelling to use in this reminiscent context is 'heyday', not 'hayday' (and not 'hey day' or 'hay day' either).
Hay picture from Shutterstock
The Macquarie Dictionary defines a 'heyday' as "the stage or period of highest vigour or fullest strength", which is pompous but clear. The phrase apparently originates in the expression 'high day', referring to a religious festival. Whatever their belief system, people who write "back in my hayday" presumably couldn't spell back then either. Fortunately, it's never too late to learn. Accuracy matters.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.