That's all the homework UNTIL July 28th
When we say until July 28th , do we count day 28 as well? or does it say all assignments were marked before day 28?
While we are it , do we say day 28th or day 28?
This means that before July 28th, no more homework will be assigned (or due, depending on context).
Until, From OLD
up to the point in time or the event mentioned
And it we use: day 28, or the 28th day, or July 28th, or 28 July (in AmE).
The preposition until in this situation means there will be no homework on all the days from now (the time of speaking) right up to the stated day the 28th. On the 28th homework will be given again.
The instructor can use the shortened date version (the 28th) but only if the students are aware that the instructor is referring to the 28th of a specific month.
The instructor might cause some confusion if she states that a very big assignment is due on the 28th if the day she says this is the 26th. Students may want to know if this is the 28th July or the 28th of August since August is about to start.
I hope that helps.