John,
I have been sloppy in the past and not included a date in every citation.  Now I have thousands of citations which I need to edit manually.
Where I have a number of citations from a single source with a common single date, e.g. a particular census, is there a way to add that date to all the citations at one go?
Not that I know of, but ... if your citations are empty, i.e. you have no notes in them with census texts or things like that, you can use the merge citations tool:

https://www.gramps-project.org/wiki/index.php?title=Gramps_3.4_Wiki_Manual_-_What%27s_new%3F#Citations_.28formerly_Source_References.29

When you do that, all citations that link to the same source and have the same attributes are merged into one, so that you only have one to add a date to, per census.

Please make a backup before doing this, and set the check mark so that citations with notes are not merged. Also note that once you've done this, you can not add notes with census texts for different families, because you only have one citation left per census year, so any note will affect all families.

regards,

Enno