Judging by the tags on the question, I'm guessing your question is related to JIRA Service Desk and SLA targets. If not, this answer will be quite off-topic .
In your Service Desk project, go to Project Settings and select SLAs. If you look at any SLA (Time to first response in the screenshot example below), business hours are specified through calendars. If you want to change the business hours, modify the existing calendar or create a specific calendar of your own.
To do so, click Calendars (marked with the red rectangle on the left, but depending on your Service Desk version, this may be somewhere else on the screen):
Screen Shot 2016-10-23 at 13.09.55.png
Then create a calendar of your own by adding/removing days and hours in line with your requirements. You can also add non-working days.
Screen Shot 2016-10-23 at 13.11.58.png
When you're done, apply the new calendar to your SLA.
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