In reply to Blizzard:
Unless you're planning a ski/sled journey, the only place that will be OK in March-Apri is Nepal. Partly because of the weather, partly because of the infrastructure of lodges/trails/transport for the main trekking areas.
March can be colder than January, according to some Sherpas, but there is a lot of annual variability. Some winters it doesn't snow much, or snows late. Sometimes the ground is bare in March, sometimes high passes are blocked through April. Some passes groups will force a track through (Thorung La), some stay blocked until they melt.