My partner works in a UK clothing store whose policy is to deny ALL holidays during December and January. This means for as long as she is in this job, we can never take a Christmas/New Year holiday.
Not knowing if this is a strict policy or not, how can best ask for time off during this "forbidden" period?
As it seems like these particular days are very important for the business, it might well be part of the job to cope with working Christmas. Just asking unofficial to see what possibilities there are is one way to start.
It's always possible to try to negotiate. If she has this time off (that she is expected to work), what can she give up in return that is equally valuable to the company? Summer vacation? Extra weekend work? Or something else extra that is makes up for the time off.
The main problem for the management to make exceptions from such a policy is that everyone might start to ask for exceptions and then it's easier to say no to everyone.
Working as a contractor is another option, where it might be easier to set the rules than for a regular employee. If it's possible in this particular case, is another question.
If your partner is very good at their job, and is in a field where being very good is important, you could try the technique used by Bill Mason to get the entire summer off (so he could canoe) every year, while holding jobs (graphic design I think) that typically gave two weeks a year, maybe 3 of vacation:
quit your job (with as much notice as you want to) at the start of the time you want off. When you return from it, apply to various places including your old employer.
The risk is that no-one will hire you afterwards. That's why you need to be good, and being good needs to matter. This is perhaps too extreme a thing to get two or three days off, but if you'd like several weeks to go away somewhere for the holidays, it might work.
I can see your employer's point in not allowing time beyond the statutory days, if your partner is in a field where it's busier at that time, and competition for the time off could cause tensions among the staff. Perhaps asking for a "leave of absence" would also be a strategy.
It's important to be able to negotiate in these situations. Is there another employee at a similar level who could cover for each other on different holidays? Both may need to plead the case to the employer that one can handle it.
A second step may be to ask for part of the day off or a shift in hours. This would prevent a lot of traveling, but at lease she could be there at a crucial time of a holiday.
At least in the US, the retail industry is notorious for hiring temporary employees to cover the additional hours since many make up to 85% of their sales during this time of year. Similar to trying to get the night off in the restaurant industry.
Keeping the context in mind, as to why, for some industries like retail, holiday period is "forbidden" for leave.
Assuming that your wife knew it before she joined this industry.
Assuming that she has strong reasons to ask for leave given the precedent it would set and impact it would have on her co-workers,working over the holiday period.
I would very nicely put those reasons forward along with a proposal to make up for the lost effort down the line, if required.
