Previous_Value_as per Day_range

Create a Numeric Month calculated Column

Create a calculated column in your table to convert month names into numbers

Month_Number =
SWITCH(
'YourTable'[Month],
"Jan", 1, "Feb", 2, "Mar", 3, "Apr", 4, "May", 5, "Jun", 6,
"Jul", 7, "Aug", 8, "Sep", 9, "Oct", 10, "Nov", 11, "Dec", 12,
BLANK()
)

Modify Your Measure to Use Month_Number

Prev_Value =
VAR CurrentYear = SELECTEDVALUE('YourTable'[Year])
VAR CurrentMonth = SELECTEDVALUE('YourTable'[Month_Number]) -- Use the numeric month column
VAR CurrentDayRange = VALUE(SELECTEDVALUE('YourTable'[Day_Range])) -- Ensure numeric comparison

RETURN
CALCULATE(
[Avg_Value],
FILTER(
ALL('YourTable'),
('YourTable'[Year] = CurrentYear && 'YourTable'[Month_Number] = CurrentMonth && VALUE('YourTable'[Day_Range]) < CurrentDayRange) ||
('YourTable'[Year] = CurrentYear && 'YourTable'[Month_Number] < CurrentMonth) ||
('YourTable'[Year] < CurrentYear)
),
LASTNONBLANK('YourTable'[Day_Range], [Avg_Value])
)