March is marked by gradual transition from winter to spring. The ground is likely to be covered with snow well into the middle of the month, but closer to twenties it must be melted. Air temperature rises slowly, and days get longer.
April is a shoulder-period between winter, and summer, the pure spring month indeed, when one might enjoy moderate air temperatures, and sunny days, as well as some rain, and wind.
When March resembles winter and April deserves to be called a spring month, May is already summer-like. Warm air temperature, and hence the abundance of outdoor activities result in May being the start of the high season.