"I swim every week."
When is it correct to use the particle "は" after a time phrase? I often see "今日は..." but in this sentence "毎週は泳ぎます。" was marked incorrect. Is this an oversight in the course or are there certain situations where "は" is not used?
は marks something as the topic of discussion, so it really depends on whether that particular time is intended to be the topic of discussion or not. So, it's not really an oversight, but more certain sentences focusing more on the time and certain sentences focusing less on the time. As far as I know, は cannot be used for unknown times like "何日". Also important to mention is that に cannot be used with relative times, such as 今日, 昨日, or 明日.
For anyone reading this in the future, temporal nouns are a bit weird in the sense that sometimes can act directly on a verb as it were an adverb, this behavior falls on the category of adverbial nouns. So if you see a temporal noun like 毎週、月曜日、来月 directly added onto the start of the sentence you can assume that is being mentioned to affect the verb. Sometimes this is also just a particle being skipped on speech (duo sometimes adds a comma in between) but if you compare legal documents with speech you realize that this particular behavior of temporal nouns is accepted as grammatically correct by japanese people. The difference between declaring the time as a topic (using は) is the stress on the time you are mentioning, otherwise is just mentioning the change of the verb in a temporal line.
毎月（まいつき or まいげつ）Every month/monthly