Common day and month slips
Ten minutes on the topic's own four common mistakes: a day name capitalized the English way, a feminine day left in its dictionary form instead of the accusative, a month left in its dictionary form instead of the -у ending, and недеља (Sunday) forgetting the same accusative ending субота takes. Build sentences about being late, catching each slip in turn, before the unit's own listening and reading put the whole topic to work.
The part of the grammar page this lesson drills — read it here, then use it.
Days of the week
The seven days: понедељак (Monday), уторак (Tuesday), среда (Wednesday), четвртак (Thursday), петак (Friday), субота (Saturday), недеља (Sunday). Four are masculine consonant-ending nouns (понедељак, уторак, четвртак, петак); three end in -а and are feminine (среда, субота, недеља).
To say on a day, use у + accusative: masculine days don't change (у понедељак), feminine days take -у, just like the feminine nouns in cases-introduction-a1 - среда → среду, субота → суботу, недеља → недељу.
Months
The twelve months: јануар, фебруар, март, април, мај, јун, јул, август, септембар, октобар, новембар, децембар - all masculine, and all international/Latinate names.
To say in a month, use у + a special -у ending: у јануару, у марту, у септембру. This ending belongs to the locative case (covered fully in a later lesson) - for now, just learn у + [месец]у as one fixed chunk.
Special cases
Недеља means both Sunday and week - context tells them apart. A less common synonym, седмица, exists specifically for week.
The one exception is positional, not lexical: a day or month name is capitalized only when it happens to start a sentence, exactly like any other word - never in a date or a title.