The Project Instructions do say "When there is no vital date, but the vital date can be reasonably calculated from the publication date with the information given in the entry, key the calculated date into the event date fields." Therefore I would key the date as given in your example.
I've found it quite helpful to have a historical calendar open for those examples that say eg "last Thursday".
Edited - I've just seen on the "Discussion" page of the Wiki for this project that maybe looking at historic calendar online is maybe going a step too far. I've keyed several of these so not sure whether the Reviewer will delete this or not.