I have a September birthday.
I hate it, and I’ve always hated it. I had a cousin born at the end of July, but in the same school year. If I did anything well at school, it was always pointed out that ‘she’s a year older than ‘Emily’, really.’
I hated the way that for the duration of the school holidays my parents would say I was the age I would be in September.
I hated the way that my birthday happened almost as soon as the school year began.
These are all really super petty reasons and I know the real reason is that I hate turning 30, 40, whatever, almost a year before most of my peers.
I know this is ridiculous. I know people have good reasons for avoiding some months, mostly Christmas and July/August because of academic disadvantages, but ideally I’d really want a child born between March and May.
Maybe thinking about it it’s more that I don’t want my child to be like me 
This is a totally hypothetical child, by the way.