PowerShell - next day - sql

Question

I am trying to look into some past days to see if they are weekend or bank holiday. If they are failling into weekend, I am trying to run the query with the subsequent Monday of if bank holiday, trying to run it with next day.
The code is below.
function Check-BankHoliday
{
param
(
[Parameter(Position=1, Mandatory=$true)]
$DayChk
)
$JAMSHost="ukjam01apwpd"
if(!(test-path "JD:\")){$null=New-PSDrive JD JAMS $JAMSHost}
$CalendarName = "Default"
$DateTypeName = "BankHoliday"
$BHDates = Get-ChildItem JD:\Calendars\$CalendarName\$DateTypeName | Select-Object StartDate
$BHDayChk=$false
ForEach ($Date in $BHDates)
{
$BHDayDate=$Date.StartDate.ToString("dd/MM/yyyy")
if($BHDayDate -contains $ImpDateChk)
{
$BHDayChk=$true
}
}
Pop-Location
Return $BHDayChk
}
$lastquarter="select convert (char,(SELECT DATEADD(dd, -1, DATEADD(qq, DATEDIFF(qq, 0, GETDATE()), 0))), 23) as lastquarter"
$lastquarter_day=Invoke-Sqlcmd -Query $lastquarter -ServerInstance "UKWAR01APWPD\WHERESCAPE" -Database RJISDWH
$lastquarter_day= $lastquarter_day.lastquarter
$lastquarter_day = $lastquarter_day.trim(' ')
$lastquarter_day=(Get-Date $lastquarter_day).DayOfWeek
write-host $lastquarter_day
$BHDayChk_lastquarter=Check-BankHoliday $lastquarter_day
if ($BHDayChk_lastquarter -OR $lastquarter_day -eq "Sunday" -OR $lastquarter_day -eq "Saturday")
{
Write-Output "$lastquarter_day had been set as BankHoliday or weekend JAMS Calendars and Changing this to LastWorkDay"
#$lastquarter_day= ConvertTo-Date "today -1 workday" -server $JAMSHost
$lastworkday= ConvertTo-Date "today -1 workday" -server $JAMSHost
$lastquarter=$lastworkday.AddDays(1)
}
else
{
Write-Output "$lastquarter_day is not a BankHoliday or weekend in JAMS Calendars"
}
$SQLquery = "
declare #lastquarter varchar = '$lastquarter_day_date'
declare #sql nvarchar(1000)
set #sql = 'select bp1.sedol, b.benchmark_name, b.description,
bp1.price as '' ' +convert(char,#lastquarter,23) + ' ''
FROM
dbo.ds_benchmark_prices bp1
left join [RJISDWH].dbo.ds_benchmark b on b.sedol=bp1.sedol
where convert(char,bp1.price_date,23) = ''$lastquarter_day'' -- last quarter end
order by b.sedol'
exec (#sql)
"
$lastquarter is returning 2022-12-31 which is a weekend ( Saturday) and if condition below is capturing that being Saturday successfully too.
if ($BHDayChk_lastquarter -OR $lastquarter_day -eq "Sunday" -OR $lastquarter_day -eq "Saturday")
Howevever, how would I add day to Saturday so it runs on the following Monday . I have tried
$lastworkday= ConvertTo-Date "today -1 workday" -server $JAMSHost
$lastquarter=$lastworkday.AddDays(1)
write-host $lastquarter
and that would return 20/01/2023 00:00:00
Below would not work
$lastquarter=$lastquarter.AddDays(1) or $lastquarter=$lastquarter_day .AddDays(1)
I get following error
Method invocation failed because [System.String] does not contain a method named 'AddDays'.
At line:47 char:3
$lastquarter=$lastquarter.AddDays(1)
+ CategoryInfo : InvalidOperation: (:) [], ParentContainsErrorRecordException
+ FullyQualifiedErrorId : MethodNotFound