The Swedish prime minister Stefan Löfven resigned on June 28th, after losing a vote of confidence in the parliament. Now there will be discussions between the speaker of the parliament and all party leaders in the search for a majority coalition. The two major blocks are very tightly matched at this point at 175-174. After the election in 2018 it took 134 days to form an agreement between the parties that have been in majority since then.

The speaker will make a suggestion to the parliament when he feels he has found a candidate for prime minister that the majority will accept. There is no formal time limit for this process. If at least 175 out of the 349 members of parliament vote against the candidate, the speaker has to come up with a new suggestion. After four votes where each candidate loses, an extra election must be announced, and the election will need to take place within 3 months.

Sources: https://www.thelocal.se/20210628/swedish-prim…

https://www.thelocal.se/20210621/sweden-stefa… (before the resignation)

Will Sweden have an extra election before the end of December 2021?

The question resolves positively if there is a general election for parliament in Sweden on or before December 31 2021. The final results need not be ready before this date.

In all other cases this question resolves negatively (e.g. there is no election because a new PM is elected by parliament, or there is an election on or after January 1 2022).

Pre-votes (mail votes) may be sent beforehand, but for positive resolution there needs to be an actual "voting day" where physical voting locations are open to the public, before December 31.