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What industries reject your vacation time the most

It's mid-September, which means your human resources department is probably planning vacation calendars for 2016.

If there's specific time you're looking to take off, it's best to submit your hours as far in advance as possible; you're competing with everyone else in your company for those prime weekends.

But the industry you're in can have a huge effect on whether your boss will allow the time off or reject your request, according to data provided to CNBC from Replicon, a company that makes time sheet and expense-management software.

"Health care jumped out immediately," said Jose Gaona, vice president of product strategy at Replicon. "That is a very complex business because they're 24/7. There's a shortage, still, of registered nurses, and there's all sorts of scheduling that has to happen."

Replicon looked at 664 businesses that use its software during the period January, 2014, through June, 2015, for the data.

More than 17 percent of the hours rejected during the period studied were in health care, the highest of any industry, on a weighted average basis. That means that based on the number of firms that use Replicon's software from each industry, employees of hospitals, clinics and health-care labs were the least likely to have their time-off requests approved.

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It may come as something of a surprise that it's not blue-collar industries such as manufacturing and construction where employees have the most trouble getting time off. But it makes sense if you consider how the industries operate around production schedules, Gaona said.

"They're kind of on timelines, and they're producing the right amount," Gaona said. "They know they're on the job for a certain amount of time."

The time of year you're looking for time off can also have a big effect on your chances of taking the time off, and it's not what you might think.

Turns out October is the most competitive month to get time off, according to the data, when companies reject an hour for every 88 time-off hours submitted. That's roughly the same as for the month of June, the first month of summer and when many parents find themselves in need of PTO to care for kids no longer in school.

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"It's really tough for businesses at that time to allow for that time off," Gaona said, "because everyone wants it."

October ranks so high because of fall break, Gaona said. Parents either want to take the time to travel with their kids or they can't find child care and need to stay home. Likewise, April and May come in after October and June as the most-rejected time off, likely because of spring break.

"It's a very synchronized swim that happens with taking time off," Gaona said.