In your workplace, how are vacations granted when more than one person requests the same time period off?

I’m not asking because I’m having a problem, I’m asking because in my line of work this is a common issue (and it’s vacation time now so the topic is coming up more).

If more than one person requests the same days or weeks off, does it go to the person with the most seniority? Or the person who put in the request first? Or some other way?

If it goes to the person (or people) with the most seniority, that could be a problem because chances are the same people will get the prime weeks off year after year (for example the days between Christmas and New Year’s). If it goes to the person who put in the request first, is there a “no earlier than” date to put the request in? For example, if they want to put in for the week after Christmas, can the request be put in no earlier than June or October or two months prior to the date requested?