To produce 0 values I would recommend the following approach:
Step 1: Create a lookup
## expected_weekdays.csv
date_wday,count
sunday,0
monday,0
tuesday,0
wednesday,0
thursday,0
friday,0
saturday,0
## transforms.conf
[expected_weekday_lookup]
filename = expected_weekdays.csv
Step 2. Use the lookup in your search
index="my_index" | bucket span=1d _time | stats count as countActions by User, _time, date_wday | append[| inputlookup append=T expected_weekday_lookup | rename count as countActions] | sort User, date_wday | stats avg(countActions) as Weekday_average by User, date_wday
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