I've got a column with dates in it - in another column I'd like a formula that shows the most recent 28th of the month (or last working day beforehand if necessary/possible).

For example, if it showed any date from 28th September to 27th October 2015 I'd want the formula to result in 28th September 2015.
The 28th November is a Saturday, so I'd want dates from 28th October to 26th November to show 28th October 2015, dates from 27th November to 27th December to show 27th November, and so on.

Any help appreciated