list all dates from previous month

Just Google a calendar table and add it

https://github.com/camiolo/Power_Query/blob/main/Power%20Query%20Template.xltx

In my Excel file, I have a Function Month_Generator_Loop.

Here's a code of that function to generate Month's between two days.
It should also work if you Replace Text .AddMonths( with .AddDays(

let Input = (Start_Month as date, End_Month as date, Empty_List as list) => let Step_1 = List.Combine({Empty_List, {Start_Month}}), Step_2 = if Int64.From(Start_Month) < Int64.From(End_Month) then Month_Generator_Loop(Date.AddMonths(Start_Month,1), End_Month, Step_1) else Step_1 in Step_2 in Input

This is my one-line Table Calendar:

​

= #table(type table[DateKey=date], List.Transform({Number.From(#date(2020,1,1))..Number.From(#date(2020,12,31))}, each {Date.From(_)}))

M is a functional language and practicing with little things like this is how you get good at it!

let

Now = DateTime.LocalNow(),

PreviousMonth = Date.AddMonths(Now,-1),

StartOfPreviousMonth = Date.From(Date.StartOfMonth(PreviousMonth)),

DaysInPreviousMonth = Date.DaysInMonth(StartOfPreviousMonth)

in

List.Dates(StartOfPreviousMonth,DaysInPreviousMonth,#duration(1,0,0,0))