I'm afraid that to expand the date interval in Power Query we need to write a line of M code.
This is a small sample that creates a sample table with the columns of the table in your question. I used different value to keep the example simple.
The idea is to expand the dates interval creating a M List, containing the interval of dates expanded. Then to use this list to create the new rows with the new column "Date".
The last step removes the "Start Date" and "End Date" columns
This code can be directly pasted into the advanced query editor for a new blank query.
let
Source = #table(
type table
[
#"ID"=number,
#"Start Date"=date,
#"End Date"=date,
#"Category"=text
],
{
{1,#date(2020,1,1),#date(2020,1,2), "A"},
{2,#date(2020,1,10),#date(2020,1,12), "A"}
}
),
SourceWithList = Table.AddColumn(Source, "Date",
each List.Dates([Start Date], Duration.Days([End Date] - [Start Date]) + 1, #duration(1, 0, 0, 0))),
#"Expanded DateList" = Table.ExpandListColumn(SourceWithList, "Date"),
#"Removed Columns" = Table.RemoveColumns(#"Expanded DateList",{"Start Date", "End Date"})
in
#"Removed Columns"
The Source statement is just needed for the example, to create the starting table.
The SourceWithList is the M code to be written: it adds a column using the function Table.AddColumn(), and creates the new column using the function List.Dates().
This function requires the start date, the duration and the step interval.
The duration is computed with the function Duration.Days() that returns the difference between two dates as number of days.
To create the #"Expanded DateList" step it's possible to use the Power Query interface clicking on "Expand to new rows" in the column menu. The screenshot I took are in Power BI, but the Power Pivot interface for Power Query is very similar.
![expand to new rows](https://i.stack.imgur.com/vMq32.png)
Then remove the "Start Date" and "End Date" columns by selecting the column and clicking on "Remove Columns"
![remove columns](https://i.stack.imgur.com/X1o3V.png)