I think if you feel comfortable with it, your parents feel comfortable with it and are well enough to care for a new baby, and your baby seems to adjust to it fine and without any problems cropping up, then I don't see any problems with it.
Me personally, I couldn't do it. I've only left my daughter with anyone but my husband once since she was born. My mom came to visit from the U.S. and watched her while we had lunch at the pub about a 30 second walk from our house. I lasted about 1.5 hours before I cried and had to come home. That said, I don't especially trust either of our sets of parents to watch her anyway, at least not as a baby. Husband's mum and partner would probably just leave her to cry in a room somewhere while they drank wine and ignored her (they don't really like kids). And my mom, she's great, but she's just getting a little too old to be on point all the time with a tiny baby. I'm hoping when our daughter is older and more self-sufficient, I'll feel differently, but for now, I'm not comfortable with it.
But I think we have to keep in mind that it's an incredible luxury in our culture to be able to spend so much time with our children. There are many people (especially in the U.S. where I'm from) who are back to work full-time, even having to go away on business trips, from 6 weeks pp. If I still lived there, I would have had to and would have spent very little time each week with my new baby. Not saying it's ideal, but it's reality. I'd say certainly an overnight with grandparents once a week while spending the rest of the week close to mum is better than having to put your 6 week old in childcare 40 hours a week, which is what most of my U.S. friends have had to do. I know people have to do it and it's often not a choice, but I'd still feel more comfortable with one than the other at that age. And there are plenty of cultures where grandparents co-parent and are even primary caregivers while parents work or live away. Maybe that's not ideal for some people, but I don't think it's terribly damaging either.
You know what feels right for you and your family and no one should tell you how to parent. I think enjoy your night out. We've not had one yet and I wish we could!