Lag for same day last month with fix for missing end of month days

FixLagSameDayLastMonth.sql

/* don't lock, be deadlock victim if needed  */
set transaction isolation level read uncommitted
set deadlock_priority -10

/* generate sales data */
declare @SalesByDay as table (
	  [SalesDate] datetime not null primary key
	, [TotalSales] decimal(19,2) not null
)
declare @mindate date = '20200101', @maxdate date = '20200630'
;with Dates([Date]) as (
	select @mindate
	union all
	select dateadd(day, 1, [Date])
	from Dates 
	where [Date] < @maxdate
)
, SalesByDay([Date],[Sales]) as (
	select *, CAST(RAND(CHECKSUM(NEWID()))*100000 as decimal(19,2))
	from Dates
)
insert into @SalesByDay
select * from SalesByDay
option (maxrecursion 0)

/* show our example and fix */
select
	  [SalesDate] = cast(format([SalesDAte],'yyyy-MM-dd') as varchar(10))
	, [TotalSales]
	, [SalesPrev] =
		lag([TotalSales]) over (partition by day([SalesDate]) order by [SalesDate])
	, [Fixed] =
		case
			when day([SalesDate]) <= day(eomonth(dateadd(month,-1,[SalesDate])))
			then lag([TotalSales]) over (partition by day([SalesDate]) order by [SalesDate])
		end
from
	@SalesByDay
where
	day([SalesDate]) in (29,30,31)
order by
	SalesDate