Select-Object returns a custom PSObject with just the properties specified. Even with a single property, you don't get the ACTUAL variable; it is wrapped inside the PSObject.
Instead, do:
Get-Date | Select-Object -ExpandProperty DayOfWeek
That will get you the same result as:
(Get-Date).DayOfWeek
The difference is that if Get-Date returns multiple objects, the pipeline way works better than the parenthetical way as (Get-ChildItem)
, for example, is an array of items. This has changed in PowerShell v3 and (Get-ChildItem).FullPath
works as expected and returns an array of just the full paths.