This Sunday up in London the Roundhead Association of the ECWS are marching to celebrate the anniversary of the Regicides who were hanged drawn and quartered in 1660. What that is all about is well explained on this blog here with times etc. Should be fun if you are in the capital.
Image A Roundhead by John Pettie
If you're wondering where the term Roundhead comes from here's what the wiki says
Origins and background
"Roundheads" appears to have been first used as a term of derision toward the end of 1641, when the debates in Parliament in the Bishops Exclusion Bill were causing riots at Westminster. Some, but by no means all, of the Puritans wore their hair closely cropped round the head, and there was an obvious contrast between them and the men of courtly fashion with their long ringlets. One authority said of the crowd which gathered there, "They had the hair of their heads very few of them longer than their ears, whereupon it came to pass that those who usually with their cries attended at Westminster were by a nickname called Roundheads." According to John Rushworth (Historical Collections) the word was first used on 27 December 1641 by a disbanded officer named David Hide, who during a riot is reported to have drawn his sword and said he would "cut the throat of those round-headed dogs that bawled against bishops".
However, Richard Baxter ascribes the origin of the term to a remark made by Queen Henrietta Maria at the trial of the Earl of Strafford earlier that year; referring to John Pym, she asked who the roundheaded man was.
The principal advisor to Charles II, the Earl of Clarendon (History of the Rebellion, volume IV. page 121) remarked on the matter, "and from those contestations the two terms of 'Roundhead' and 'Cavalier' grew to be received in discourse, ... they who were looked upon as servants to the king being then called 'Cavaliers,' and the other of the rabble contemned and despised under the name of 'Roundheads' ".
Ironically, after Anglican Archbishop Laud made a statute in 1636 instructing all clergy to wear short hair, many Puritans rebelled to show their contempt for his authority and began to grow their hair even longer  (as can be seen on their portraits), though they continued to be known as Roundheads. The longer hair was more common among the "Independent" and "high ranking" Puritans (which included Cromwell), especially toward the end of the Protectorate, while the "Presbyterian" (i.e. non-Independent) faction, and the military rank-and-file, continued to abhor long hair. By the end of this period some Independent Puritans were again derisively using the term Roundhead to refer to the Presbyterian Puritans.