Pyth, 7 6 bytes

l-6.d9

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Explanation

   .d9 # Get the current day of week (0 = Monday, 6 = Sunday)
 -6    # Subtract 6 from the day
l      # Try to calculate the log base 2 of the result of the previous operation
       # raising a "ValueError: math domain error" on sundays

Pyth, 8 7 bytes

 l-6.d9

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Explanation

    .d9 # Get the current day of week (0 = Monday, 6 = Sunday)
  -6    # Subtract 6 from the day
 l      # Try to calculate the log base 2 of the result of the previous operation raising a "ValueError: math domain error" on sundays
        # there is an extra space at the start, to supress the output on the other days

Pyth, 7 bytes

/1-6.d9

Explanation

    .d9 # Get the current day of week (0 = Monday, 6 = Sunday)
  -6    # Subtract 6 from the day
/1      # Divide 1 by the result of the previous operation
        # raising a "ZeroDivisionError" on sundays

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###Another 7 bytes solution

.(U6.d9

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Explanation

  U6    # Create a list with 6 elements
.(  .d9 # Try to pop the 7th element on sundays
        # raising "IndexError"

Pyth, 7 6 bytes

l-6.d9

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Explanation

   .d9 # Get the current day of week (0 = Monday, 6 = Sunday)
 -6    # Subtract 6 from the day
l      # Try to calculate the log base 2 of the result of the previous operation
       # raising a "ValueError: math domain error" on sundays

Pyth, 7 bytes

/1-6.d9

Explanation

    .d9 # Get the current day of week (0 = Monday, 6 = Sunday)
  -6    # Subtract 6 from the day
/1      # Divide 1 by the result of the previous operation
        # raising a "ZeroDivisionError" on sundays

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Pyth, 7 bytes

/1-6.d9

Explanation

    .d9 # Get the current day of week (0 = Monday, 6 = Sunday)
  -6    # Subtract 6 from the day
/1      # Divide 1 by the result of the previous operation
        # raising a "ZeroDivisionError" on sundays

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###Another 7 bytes solution

.(U6.d9

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Explanation

  U6    # Create a list with 6 elements
.(  .d9 # Try to pop the 7th element on sundays
        # raising "IndexError"