Well, I prefer it not to have been rained on at all .....but having said that, we did get some last year (a miracle given the wet summer!) and it rained overnight, but fortunately it was in big rows, so only the top got wet, and we just wuffled it through again.
How long had it been cut? And had it been turned? Was it nearly dry before the rain? If its just been cut, and then it rains, it does not worry me, its when its been turned, and nearly ready, then gets wet, not good. But, if its just rained on the once, and gets dry ok, should be ok.
Was wanting mine cut, but farmer said to wait, as he heard the long range weather forecast.....or I too would have been looking at wet hay. Been very hot and sunny here today, and now its raining. Had quite a bit of rain on Saturday, and yesterday morning, then it brightened up. Not ideal hay making weather though.
Good luck, hope you get your hay. Hate to see wasted hay.