When you summarize periods of utilization, there are two basic options:
Calculate the average: sum all values and divide by their number. Beware - averages don't properly represent uneven load and may hide bursts with congestion behind comparatively low average utilization. However, averaging gives a good estimation of overall data volume. Note that with a low frequency of ifin/outoctets sampling (less than perhaps one sample per minute), you're already averaging.
Find the maximum: find the highest value and drop all others. In contrast, this shows peak loads very well but allows no estimation of actual data volume or overall actual load. In order to see the peaks you need a high sampling frequency (reasonably more than one sample per minute).