Kedarkantha is best visited during winters and the reason is that you would get to see a snow-covered peak that resembles heaven. Otherwise one can visit the Kedarkantha in summers too but it loses its snow-covered lustrous layer in the summer and turns into a normal trek.
Kedarkantha is a good trek all round the year. It can be trekked both in summer and in winter, however it is known as the best winter trek, as it provides a beautiful view of snow and snow covered forest. So, the choice is yours