Any voter may vote in advance at any Citizen Service center in Denmark from Tuesday 6 weeks prior to election day (i.e. 10 October, 2017 re. the upcoming elections in 2017) and no later than the Friday preceding election day (i.e. Friday 17 November, 2017). Remember to bring some ID.
Advance voting can also take place at any Danish diplomatic representation abroad (embassies and consulates) from three months prior to election day, and from three weeks prior to election day also in hospitals for hospitalised voters, at care homes and sheltered housing etc., in prisons etc. for detainees, and at home upon application for voters who on account of illness or disability are unable to turn up at a polling station on election day.
If you have voted in advance, you cannot vote on election day.