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The UK Supreme Court is due to rule on whether the decision to suspend Parliament, only days after MPs returned to Westminster, was legal.
But why was Parliament shut down in the first place?
How did the prime minister close Parliament?
The official term for shutting down Parliament is "proroguing".
MPs do not vote to prorogue - it's a power that rests with the Queen, done on the advice of the prime minister.
So, it is within Boris Johnson's gift to ask the Queen to shut Parliament.
However, even though the Queen agreed to the request, legal proceedings were brought against the government.
While prorogation is normal, the timing of it in this case is "clearly hugely controversial", says Maddy Thimont-Jack, of the Institute for Government think tank.
Why is it controversial?
Normally, after a period of prorogation, Parliament reopens with a Queen's Speech. This is when the government outlines its priorities for the upcoming year.
Usually, this process is extremely straightforward. In fact, the House of Commons Library says proroguing has been a formality in the UK for more than a century.
But the decision to prorogue - just weeks before the UK's scheduled departure from the EU - brought the Queen into the Brexit dispute.
The government defended its action, saying it had nothing to do with Brexit. It says proroguing Parliament will allow the PM to outline plans for domestic policies like NHS funding.
Opponents, on the other hand, say it is a ruse. The real reason Parliament was shut down, they argue, was to minimise the opportunities to block a no-deal Brexit.
However, despite having less Parliamentary time, MPs succeeded in passing a law that seeks to extend the Brexit deadline.
How often does prorogation happen?
Parliament is normally prorogued once a year for a short period - usually in April or May.
During this time all business stops and most laws that haven't completed their passage through Parliament die a death.
MPs keep their seats and ministers remain in position - but no debates and votes are held in Parliament.
This is different to "dissolving" Parliament - where all MPs give up their seats to campaign in a general election.
It is also different to a recess - a break in the Parliamentary session - which was due to take place this year from roughly 13 September to 8 October.
How long will be Parliament be prorogued?
This year, Parliament is due to be suspended for 24 working days before the new Queen's Speech on 14 October.
That's much longer than usual. In 2016, for example, Parliament was closed for four working days, while in 2014 it was closed for 13 days.
Could prorogation be stopped?
Two of the UK's highest courts, one in England and one in Scotland, have already looked at whether prorogation was legal - only to come to opposite conclusions.
The matter will now be settled by the UK Supreme Court.
It heard two appeals over three days, one from the anti-Brexit campaigner and businesswoman Gina Miller, and one from the government.
Ms Miller was appealing the English High Court's decision to reject her challenge to prorogation.
The government, on the other hand, was appealing the ruling from Scotland's Court of Session that the prorogation was "unlawful" and had been used to "stymie" Parliament.
The challenge to prorogation in the Scottish Court was brought by 75 parliamentarians, including SNP MP Joanna Cherry.
The Supreme Court will make its ruling on Tuesday.
So could the government be ordered to reopen Parliament?
If the Supreme Court judges rule that prorogation is legal, then Parliament will remain shut until 14 October - unless the government advises the Queen to change the date.
If Ms Miller's side wins, it's still unclear how and when Parliament would resume - whether MPs could simply return to the Commons, or if they would need to be recalled by The Queen, on the advice of Mr Johnson.