Selling in May is part of a well known saying in the investment sector.

'Sell in May but remember to come back in September'

You can compare this with other factors some people say influences the stock market.

E.g. More black/grey suits would indicate a negative sentiment on the market

Of course you should choose yourself if you think that these sayings can be true. In the past I've chosen to sell my shares in May because there isn't as much action going on in the summer months compared with the rest of the year and therefore don't fit with my investment strategy.