Is this what you need?
Sub MoveDownData()
Dim S1 As Worksheet: Dim rng As Range
Dim Cel As Variant: Dim EndRow As Long
Set S1 = Sheets("Sheet1")
EndRow = S1.Range("C65536").End(xlUp).Row
Set rng = S1.Range(Cells(1, 3), Cells(EndRow + 1, 3))
For Each Cel In rng
If Cel.Value = "" Then
Cel.Value = "Happy Holidays"
Exit For
End If
Next Cel
End Sub