There is an old phrase that is used on Wall Street every year. Once you have heard it you will never forget it, but you probably should. The phrase is "Sell in May and go away until Labor Day". Somewhere along the lines someone came up with the concept that the market under performs during this period and if you were to exit your positions in May and re-enter in September then you would stand to generate greater returns in the long run. The phrase has been proven to be false and is not something you should consider as a "holy grail". Sure there are some years where this makes sense but if you were to do this every year you would under perform those that just bought and held every year.
Here is what you should know. In May here in the states is typically the start of the summer time season. Most schools are out for the summer and families tend to take their vacations between May and August. If you were to look closely at the volume in the markets during this time period it is reduced almost in half so, yes we can make the argument that there are less participants. You may want to consider being less active during this time period but there is no reason you should sell your entire portfolio.
My suggestion is to take a look at all your positions and set some trailing stops to protect in case of a rapid decline. Then consider reducing some of you biggest winners and biggest losers for the summer and see if you can put that money to work somewhere else or just wait. During the summer months it is difficult to be active but it is not a reason to completely move to cash.