Here is the corrected routine to calculate the week number according to the ISO rules. This I have tested...

WINDOW 1
TEXT _monaco,9
DO
PRINT
INPUT "Year? "; year 'collect data
LONG IF year > 0
INPUT "Month? "; month
INPUT "Day? "; day
IF year MOD 4 = 0 THEN leap = 1 ELSE leap = 0'is year a leap year?
a = (year + INT(year/4) - leap + 1) MOD 7 'find weekday for Jan 1 (1 = Sat, 2 = Sun)
m = 4 - a 'find date of Monday in first week
d = day + INT(30.56 * month) - 32 + leap 'calculate day number
IF month < 3 THEN d = d + 2 'adjust for Jan and Feb
w = INT((d - m) / 7) + 1 'find number of whole weeks
PRINT "Week no:" w
wd = (d - m) MOD 7 + 1 'remainder gives weekday
PRINT "Weekday (1 = Mon, 2 = Tue):" wd
END IF
UNTIL year = 0
END