How to ask for a 9 day fortnight

[Somethink]

Any tips on asking for a 9 day fortnight? Performance is fine, minimal impact on others, but no one really does it at my work so would be pretty novel...

Does your work have a flexible working policy? This should outline the process for making the request.
Are you a parent of school age children? If yes then you have a statutory right to request flexible working arrangements and your employer can only turn that request down if they can evidence there would be a negative impact on the business in some way.
Could you have an informal conversation with your line manager first to test the water?

If a fortnight reduces from 14 days to 9 days when will Christmas be this year?

And my birthday?

Did it for years, and a four day week but both of these patterns were supported by my work and as the number of hours worked was exactly the same it caused no issues.
Would you be looking at reducing your hours or keeping the same hours but just worked over less days?
At the end of the day, if you put forward a reasonable case for moving to a nine day fortnight, they can only say yes or no.

Just did it on my return from Mat Leave after DSis suggested it. I made a flexible working request (found a template online) and it was approved pretty quickly. Due to the nature of my work it had minimal impact on the business. My line manager and his boss were very supportive, which was really helpful. I work an extra 50 mins a day and it gets me every other Friday off, and I was basically doing those hours anyway. Very happy so far!

You don't ask, you make a request in line with your flexible working policy. Don't go into any personal reasons like childcare etc.

This
most full timers where I am do a 9 day fortnight

I did this last year. I work 8:20 per day and take 40 mins for lunch. In reality I do more/less as I never want to be in the position of having to justify. It works well for me and I had no issue getting it approved - but I made sure I based my reasoning around how it would benefit the business and they fully accepted it.

I do this. I work 8-5 four days and 8-4 on the Friday and get every second Friday off. It's great. Although we've been so busy recently I've not always been able to take it.

I out in a request and as I have a disabled child it was agreed.

I'm trialling it as a reasonable adjustment but at my workplace there is a formal procedure to request it if that option is not available to you.
It gets confusing with annual leave and my last paycheck was wrong so it's not completely trouble free so far.