MonthX Unique Events= CALCULATE(DISTINCT('Incident Tracker'[Incident Name]),'Incident Tracker' [Incident Date] >= DATE(2024,1,1) && 'Incident Tracker' [Incident Date] <= DATE(2024,10,1))

If you have a separate date table and a slicer on it (to make it more dynamic) i would consider using VALUES and TREATAS in the filter argument of CALCULATE