How to handle missing data for machine learning

Question

I'm trying to come up with a data structure to predict water visibility in a lake. I have some measured samples but would like to take other features into the equation.

As an example, I would like to get weather data such as rain and temperature for the past 7 days of an event. I got this data from BigQuery which seems to work fine. But sometimes it does not find data for all the past 7 days. How would I handle such a case? What would be a good way to structure my features? I thought something like:

dayofyear,temp,water_temp,temp-1,rain-1,dayofyear-1,temp-2,rain-2,dayofyear-2,....,temp-7,rain-7,dayofyear-7,visibility

While the last one would be the field I like to predict at some point.