MySQL determine day of week when working with multiple date fields
What you're doing is fine as far as a working solution goes, and helps avoid using functions to calculate the day of the week in memory every time the query runs. But MySQL has built in functions to calculate it as well which is just as fine of a solution too.
For example, the following returns the week day index using MySQL's WEEKDAY() function:
CASE
WHEN dl.date_inserted IS NOT NULL
THEN WEEKDAY(dl.date_inserted)
ELSE WEEKDAY(dl.entry_date)
END AS day_of_the_week

If you want the name of the week day, then you can use the DAYNAME() function instead.