You can surely visit Nepal at any time of the year, but keeping in mind what activities you want to do: it’s best if you schedule it accordingly to the best time to visit Nepal. Touring cities during the monsoon can give you a tough time or not getting to see the views due to fog can be a real bummer. Therefore it is best to find out what the ideal time for your travel is and take off. Generally, we do not categorize any season as the ‘worst’, but we do consider two seasons to be the best seasons for visiting Nepal. The spring and autumn months that occur from March to May and mid-September to November are the ideal months to travel to Nepal. You will witness a pleasant climate with clear skies. For trekkers and climbers, the trails are dry and easier to walk on.
For more information on the best time to visit Nepal with graphs, pros and cons of each season visit this page.