Theme Day menus can only be used for one day in the calendar, as opposed to the Lunch category which is used to set up a menu for the week.
This article will teach you...
How to add a Theme Day menu and adding Menu Items to the new Theme Day menu.
Menu templates for students with Dietary Requirements and Allergies remain active if a Theme day menu is applied to a site/area, making it possible for these students to keep their Special Menu items.
Enable the new user interface via the toggle in the top right-hand corner
When setting up Menus you can create a Theme menu for one day only, as opposed to the Lunch category which is used to set up a menu for all five weekdays
Go to Menus > Menus > Theme Days  > click the Actions button 
Click the Add Menu  option to open the Add Menu window.
When creating a new Theme day menu give it a Description  that is easy to identify (e.g. Gluten Free Menu #2).
Set the Item types  for the Theme day. This defaults with Main and Dessert pre-selected. You can decide whether to include Vegetables, Drinks and Other items by clicking the check-boxes and click Save  when done.
To add Menu Items to the new Theme Menu select the New Theme Day  (Gluten Free Menu #2).
Users can select Items from the Menu Items  lists by dragging and dropping a Main Menu into the Main Menu  field. You can apply up to 20 Items to a Main Menu and up to 8 Menu items to a Vegetable Menu, Desserts Menu and Drinks Menu. Click Save  when done.
The Whiteboard UI has been updated to make this larger selection visible by using left and right screen scrolling. The Web based Parent Pre-selection also supports the increase in available menu items.