This seems to work as well:CREATE TABLE #SampleData (
    DayOne varchar(9) NOT NULL,
    DayTwo varchar(9) NOT NULL
);
CREATE CLUSTERED INDEX NDX_TEMP_SampleData_DayTwo_DayOne_ ON #SampleData
    (
    DayTwo ASC,
    DayOne ASC
    );

INSERT INTO #SampleData (DayOne, DayTwo)
SELECT X.DayOne, X.DayTwo
FROM (
    VALUES    ('Monday',    'Friday'),
            ('Tuesday',    'Wednesday'),
            ('Friday',    'Monday')
    ) AS X (DayOne, DayTwo);

CREATE TABLE #TwoWeeks (
    ID int UNIQUE NOT NULL,
    TheDate date NOT NULL,
    WeekDayName varchar(9) NOT NULL
);
CREATE UNIQUE CLUSTERED INDEX UCDX_TEMP_TwoWeeks_WeekDayName_TheDate ON #TwoWeeks
    (
    WeekDayName ASC,
    TheDate ASC
    );

SET NOCOUNT ON;
PRINT CONVERT(varchar(30), SYSDATETIME()) + ' - Query Start';

INSERT INTO #TwoWeeks (ID, TheDate, WeekDayName)
SELECT X.ID, X.TheDate, X.WeekDayName
FROM (
    VALUES    ( 1, '1900-01-01', 'Monday'),
            ( 2, '1900-01-02', 'Tuesday'),
            ( 3, '1900-01-03', 'Wednesday'),
            ( 4, '1900-01-04', 'Thursday'),
            ( 5, '1900-01-05', 'Friday'),
            ( 6, '1900-01-06', 'Saturday'),
            ( 7, '1900-01-07', 'Sunday'),
            ( 8, '1900-01-08', 'Monday'),
            ( 9, '1900-01-09', 'Tuesday'),
            (10, '1900-01-10', 'Wednesday'),
            (11, '1900-01-11', 'Thursday'),
            (12, '1900-01-12', 'Friday'),
            (13, '1900-01-13', 'Saturday'),
            (14, '1900-01-14', 'Sunday')
    ) AS X (ID, TheDate, WeekDayName);

SELECT SD.DayOne, SD.DayTwo, D2.ID - D1.ID AS DaysBetween
FROM #SampleData AS SD
    CROSS APPLY    (
                SELECT TOP (1) TW.ID, TW.TheDate
                FROM #TwoWeeks AS TW
                WHERE TW.WeekDayName = SD.DayOne
                ORDER BY TW.TheDate
                ) AS D1
    CROSS APPLY (
                SELECT TOP (1) TW.ID
                FROM #TwoWeeks AS TW
                WHERE TW.WeekDayName = SD.DayTwo
                    AND TW.TheDate > D1.TheDate
                ORDER BY TW.TheDate
                ) AS D2

PRINT CONVERT(varchar(39), SYSDATETIME()) + ' - Query Complete';

DROP TABLE #SampleData;
DROP TABLE #TwoWeeks;
The query portion runs in roughly 15.6 ms, and consistently.   Your mileage may vary.