If you create an ad-hoc key date work type, this can be used for any date.  It allows the user to edit the description to be what they need it to be.

The steps below explain the specific configuration for an ad-hoc key date.  For full details see Everything you need to know about Key Dates. 

1. Create an Ad-Hoc Key Date work type.
   See Create a Key Date. 
2. In the Key Date General Settings screen, on the Title & Reference click the Title Edit generators link. 
   
3. This opens the Edit Generators blade. On the Title generator, click the padlock to Override reference generator.
   
4. Set Mode to Generator and in Generator select Configurable generic reference generator. Tick Load Data Composer fields and click Link Field icon.
   
5. This opens the Context Browser.  Select Work Item > Description > Text and click the + to select.  Click Confirm.
6. In the Generation mask:  click the Generation mask help and copy the workitem.description.text tag and paste into the Generation Mask field. 
   
7. Make sure Allow user editing is ticked.  This allows the user to edit the title.
8. Save and Close.
9. When you add the Key Date to a work type (see Add a Key Date to a Work Type) make sure Allow Multiple and Add Description are both ticked.