Things had been, well, unpleasant between Charles and Parliament for some time. In fact, things between Charles and just about everyone in the country had been tense for a while. The people were upset because he’d married a Roman Catholic, dissolved Parliament, ruling on his own for 11 years (known as the Eleven Years’ Tyranny), and started instituting stupid fines to raise cash, and Scotland hated him for trying to impose Anglicanism on the country, which only served to kick off a rebellion that led Charles to invade Scotland (and get his ass handed to him). When he was finally forced to reconvene Parliament, you can bet they were thoroughly cheesed and had a list of grievances a mile long.
Charles and Parliament managed to settle into an uneasy truce for a few months, but then Charles got a little full of himself and decided to attempt to arrest five members of the House of Commons for Treason. Imagine how well that went over. The five men were hidden away, and Charles was forced to retreat, humiliated. Not long after, in fear of his safety, he fled London, and frequent letters between himself and Parliament failed to bring about a reconciliation. Seeing the way the winds blew, various towns throughout the country began declaring allegiance to one side or the other. One of those towns, Hull, refused to let Charles enter when he showed up, looking for arms to equip his men. He went to Nottingham instead, where he raised the royal standard and apparently threw down the gauntlet on 22 August.
After that, things started to move pretty fast. Armies were raised, and only a month later the first major cavalry engagement of the war, the Battle of Powick Bridge, was fought, resulting in a royalist victory. Thus began nearly two decades of war and a rather miserable existence under Oliver Cromwell. By the end of the decade the king would be dead, the royal family exiled, and Christmas officially illegal. It’s hardly surprising that, once Charles II was restored to the throne, everyone was seriously ready to party.