Probably need more details here, but you could do your base search for "errors" and have that run for the last 5 days, then pipe those results to stats and distinct count the days over which the errors existed, then trigger an alert if that distinct count >= 5.
index=foo sourcetype=bar "error" earliest=-5d | stats dc(date_wday) as days
Save As Alert then set a Trigger Condition for when days>=5.
Something like that maybe!
