You should have a Business Continuity Plan and a Critical Interruption Plan in place and all staff should know both. For salons and spas, these tend to be one and the same document.
Business Continuity Plan
Your plan of how you will continue to run your salon if one or more of your staff phone in sick.
Critical Interruption Plan
Your plan of what to do if you need to close the salon for one or more days.
What to include in each
The list goes on but in other words: plan now because it can get chaotic very quickly when things are hitting the fan.
And make sure all your staff know what to do.
The NHBF have produced a really good guide which also looks at what to do if you need to close and what rights you have as an owner here: