Looking at the SUMMARIZE formula, the first parameter is the table that we want to summarize, and the next two columns are the columns according to which we want to group our data. Note that you can group by one, two, three, or nearly any number of columns.
The next four parameters are name/expression pairs. DAX understands where these pairs start when you stop referring to column names in the table and enter a text value for a parameter denoted by double quotes. These name/expression pairs specify a column name as the first part of the pair and a DAX expression to evaluate as the second portion of the pair. We first create a column called # of Days, and the value to be returned in this column is the count of the rows from our groupings. In other words, we get the number of each weekday in each month as our value. For the second column called First Date, we return the minimum date from our groupings. In other words, we get the first date of each weekday within each month as our value.
Looking at our GROUPBY formula, the first parameter is the table that we want to group. The next two columns are the columns according to which we want to group our data. This works in the same way as the SUMMARIZE function in that you can group by one, two, three, or nearly any number of columns.
Again, with the next four parameters, these are similar to the SUMMARIZE function in that they are name/expression pairs. This works exactly like the SUMMARIZE function except that instead of referring to the original table, you must refer to a special DAX function that can only be used within a GROUPBY function – the CURRENTGROUP function. In other words, you are referring to the current row of the Cartesian product created by your grouping columns.