SharePoint Week Day Setting

SharePoint 24 Hour Calendar Display

The default SharePoint Calendar Day / Week view will not show a 24 hour calendar display. You will need to change the Site Regional Settings for this change to take place.

Using Powershell

$web = "http://sp/"
$spSite = [Microsoft.SharePoint.SPSite]($web)
$spWeb = $spSite.OpenWeb()
$spWeb.RegionalSettings.WorkDayStartHour = 360
$spWeb.RegionalSettings.WorkDayEndHour = 1080
$spWeb.Update()

The values for WorkDayStartHour and WorkDayEndHour are expressed in minutes from 12:00 AM
6AM = 6 x 60 = 360
6PM = 18 x 60 = 1080
 
Using GUI

Site > Site Settings > Regional Settings : Define Your Work Week