Past and future days (i -s, på)

Past Days: "Last" Weekday

To talk about a day in the past (like "last Tuesday"), use the preposition i followed by the weekday ending in -s.

Because this refers to a completed event, use it with verbs in the past tense.

Last Tuesday we discussed the report.
I tirsdags diskuterede vi rapporten.
Did he play in the tournament last Monday?
Spillede han i turneringen i mandags?

Future Days: "Next" Weekday

To talk about a day in the future (like "next Tuesday"), use the preposition på followed by the bare weekday.

Notice there is no -s ending here! This pairs with the present tense (which often acts as the future in Danish) or a future construction.

Next Tuesday they will discuss the presentation.
På tirsdag diskuterer de præsentationen.
What time does the tournament start next Monday?
Hvad tid begynder turneringen på mandag?

Side-by-Side Comparison

Pay close attention to the difference between i [weekday]-s (past) and på [weekday] (future).

Compare a past overnight stay versus an upcoming one:

My daughter stayed overnight in the club last Friday.
Min datter overnattede i klubben i fredags.
Next Friday my grandchild will stay overnight here.
På fredag overnatter mit barnebarn her.