The UK has two May bank holidays. The first is on 5 May this year. It's the modern version of the 1 May or May Day holiday, which in many cultures is still always celebrated on 1 May and is a public holiday on that day. In the UK until the late 1980s it was also a holiday on 1 May, but this was then changed to the first Monday after the first of May. It was said that the government of the time was doing this to break the holiday's long connection with international workers' celebrations, but in any case it was quite a popular move because it meant that people got a long weekend.
The second bank holiday is officially called the Spring bank holiday, and has no special historical significance. This one is also held on a Monday, the last of the month (in this case 26 May) and is mainly used for a short break. It's a popular holiday as this is the time of year when the weather usually starts to improve.