Match NOW() with cell formatted as date

[SamT]

You can just use Window's Scheduling to open the Workbook, then in the Workbook Open Procedure

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CheckLogin
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Very simply and with no "slip ups" testing

Sub CheckLogIn()
If Month(Now) Mod 3 = 0 (And (Day(Now) = 25 And Time(Now) > TimeSerial(23, 50, 00)) _
Or (Day(Now) = 26 And Time(Now) < TimeSerial(00, 10, 00))) Then LoginProcedure
End Sub