"I go out to the bank every month."
The basic rule is that the Kanji compounds will use the On-Yomi (音読み) reading, and that Kanji when standing on their own or with Okurigana (Hiragana suffixes use the Kun-Yomi (訓読み) reading.
Even though the rule is basic, there are many exceptions to them and that is why most of the times you’ll have to learn the Japanese words the exact way they are pronounced: for example, the 年 is pronounced「ねん」(On-Yomi) in examples like 再来年, 来年, 去年, 何年... but in 今年 it takes a sudden turn and is read as 「とし」using its Kun-Yomi.
Yes, it feels wrong somehow to put it as the topic in this sentence without context.
However, I think you can say 毎週は銀行へ出かけませんが、毎月は出かけます。 to emphasize that you do go to the bank every month, but not every week. But I suppose this is a special case. As a stand-alone sentence, it would only make sense to use 毎月は in a negative sentence, 出かけません, with the same nuance.