All seven days of the week in German are masculine. Nothing here has to be memorized one by one. You know it's a day of the week, you know the article is der.
The rule
Every day of the week is masculine. The article is der in the Nominativ. It holds for all seven days, without exception. The category determines the article, and the category never changes.
| Day | English |
|---|---|
| derMontag | Monday |
| derDienstag | Tuesday |
| derMittwoch | Wednesday |
| derDonnerstag | Thursday |
| derFreitag | Friday |
| derSamstag | Saturday |
| derSonntag | Sunday |
A note on Samstag and Sonnabend
Saturday has two names in German. Der Samstag is used across most of Germany. Der Sonnabend is the older form, still common in northern Germany and eastern Germany. Both are masculine. Both mean the same day. If you're learning standard German, Samstag is the one to know.
In conversation
Most of the time you'll hear weekdays like this → an + dem Monday (= am Montag), an + dem Friday (= am Freitag), an + dem Sunday (= am Sonntag). That am is a contraction of an and dem. That's Dativ. And Dativ is exactly what you'd expect → der becomes dem, just like every other masculine noun.
The bigger picture
Weekdays, months, seasons: all masculine → all der. It's not a coincidence: six of the seven day names literally end in -tag, der Mittwoch fell in line with them, and the months and seasons came in alongside. It does not stretch to every unit of time: die Woche, das Jahr and die Stunde go their own way. But every named point on the calendar takes der: days, months, seasons.
It's a day of the week, so it's der, all seven of them, no exceptions. The same rule covers the months and the seasons too: one category, one article.