All variables in Keyboard Maestro are TEXT.
Monday is NOT text; it is a Date property.
To use it in a boolean test against a text value you need to do something like this:
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tell application "Keyboard Maestro Engine"
set ThisDay to value of variable "DayName_Text"
set Adjustment to value of variable "Number_Adjustment_Days"
end tell
set theDate to (current date) + (Adjustment) * days
display dialog "Weekday of theDate = " & weekday of theDate & " And ThisDay = " & ThisDay
repeat until (weekday of theDate as text) = ThisDay
display dialog "Weekday of theDate = " & weekday of theDate & " And ThisDay = " & ThisDay
set theDate to theDate + 1 * days
end repeat
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