I have some diary data (Day1, Day2, Day3, Day4, Day5). If I am trying to calculate Average-Days and, for example, Day3 is missing...do I compute the average with or without Day3 included in the denominator?

I don't think I was completely clear...say it is looking at Yawns per Day...so Day1=10, Day2=0, Day3=., Day4=5, Day5=12. And I'm trying to find Average Number of Yawns per Day.

The average is the sum of the observations divided by the number of observations. If one of the observations for the week is missing how many observations have you got?

Yes, you leave day 3 out of the computation leaving you with just four values, 10, 0, 5 and 12.

I'm not sure why you're trying to divide each observation by a number as the average is caluculated from the sum of the observations (10+0+5+12) divided by the number of observations, 4 , giving 6.75