For anything other than the absolute simplest case you should have two databases. One for OLTP (the application(s)) and one for OLAP (the reporting).

If your reporting queries are hard or complex it's because you did a bad job architecting the reporting tables. Reporting queries should almost always be the simplest form of query if you designed your warehouse properly.

Days with sales is easy with a simple SUM or COUNT and GROUP BY.

Days without sales is easy if you think about this dimensionally.

  SELECT dim_days.day_of_month,
    COUNT(fact_orders.order_id)
  /*
    Select from dim_days first because you want every day.
  */
  FROM dim_days
  /*
    Outer join to the fact table to pull in the data you have and add that to your dates.
  */
  LEFT JOIN fact_orders
    ON dim_days.calendar_day = fact_orders.order_day
  /*
    Filter results by the desired range.
  */
  WHERE dim_days.month = 4
    AND dim_days.year = 2020
  GROUP BY dim_days.day_of_month;

As I continue to say:

"SQL is easy. Data is hard."