Set right filter context inside the SUMMARIZE expression

[slava_aptown]

Hi!

I am struggling with keeping and removing certain filter contexts inside a DAX expression.

I have a table on monthly granularity. Daily value column is calculated as Monthly value divided by the number of days in a month.

Despite other data is on daily granularity, I do not want to move it to daily as it will be 200 mln rows.

Month	Store	Product	Value	Daily Value  (Value / number of days in the month)
January 2024	X	X	93	93/31=3

The table has relationship with Calendar table as many-1 (Month column is formatted as the date 01.01.2024)

I write a measure that calculates Value on daily granularity as 

Daily Value * number of days chosen in the month

This is the measure. I struggle at days_chosen. In this column I want to get virtual number of days chosen in the month.
If I do not have any outside filters it should be 31 for January. If I choose only January, 1-10 it should be 10.
I have tried many combinations of ALL, ALLSELECTED, KEEPFILTER etc, but I get either virtual 1 or 31 and it does not respond to my choice in slicer. 

VAR _tbl1 =
SUMMARIZE(
    table,
    'Calendar'[Month],
    table[store],
    table[product],
    "max_value", MAX(table[value]),
    "days_chosen", 
        COUNTROWS(
            CALCULATETABLE(
                'Calendar',
                'Calendar'[Month] = SELECTEDVALUE('Calendar'[Month]),
                KEEPFILTERS('Calendar'[Day])
            )
        )
)
RETURN
SUMX(
    _tbl1,
    [days_chosen] * [max_value]
)

[lbendlin]

You may want to read this article

All the secrets of SUMMARIZE - SQLBI

Side question :  What is the number of days in "February"  ?  Which year?