Among the adjectives derived from nouns there some which do not bear Umlaut:
die Kugel - kugelig, der Monat - monatlich, usw.
But sometimes they do bear Umlaut:
das Jahr - jährlich, die Woche - wöchentlich, die Minute - minütlich, usw.
Some time ago I expected that monthly could be mönatlich but I was wrong. Is there a rule that helps by knowing when does the adjective bear Umlaut?