On the page https://www.mongodb.com/docs/rapid/reference/operator/aggregation/densify/ the range.unit list of supported values does not include "day".  I've tested and this is a supported value, but is not documented.

Current list of supported values is:

"You can specify one of the following values for unit as a string:

• millisecond
• second
• minute
• hour
• week
• month
• quarter
• year"