In the themes created after 13/02/2019 you can add an info bar with additional information, eg. “From … to … the shop will be closed. All shipments will be processed after …”
Go to Store layout> Customize> Header and general settings> Show info bar
Select this option and then set the color of the text and the background of the bar. You can also choose the option to Enable autoclose so you can set a time when the info bar should close or you can select the option Show close button so your customers can choose when to close the info bar.
You can change the bar content by clicking on the pencil icon.