The Annapurna Base Camp is situated 4.130 meters above the sea. Annapurna itself is the 10th highest mountain with an elevation of 8.091 meters.
The best time for trekking in Nepal is usually autumn, because of the weather and the best views, but at the same time, it's the peak season. However, there is a second best season less busy. Read on for more details:
March - May
The second best time for trekking here at Annapurna. April often provides clear blue skies and spectacular views to the snow capped mountains. It's mildly warm, wild flora blooming especially rhododendron but sometimes haze can obscure the mountains in May.
June - August
Heavy rainfall, monsoon season: Some tour operators offer guided trips at this time of the year. It's the low season, few hikers trek here but trails are slippery, leeches are in abundance along the trails. Due to the enormous amount of precipitation, you may be forced to rest for days.
Mid-September - November/Mid-December
Definitely the best time and the most popular trekking season with stunning mountain views, clear skies, and moderate temperatures. The downside is that during peak season, especially in October and November, flights and hotels are sometimes fully booked and rates are the highest. The trails can be busier as well. However, this isn't necessarily the case. Trekkers in October often report that they didn't have any problems getting beds at the guesthouses without reservations.
December - February
Clear skies, but mornings can be foggy. Also, snow is possible at higher altitudes, and it's getting freezing cold at night. Recommended for trekking at lower elevations only. The advantage: More solitude, as only very few others, are on the trails. Anyhow, you need a very good equipment; high fabric winter jacket and sleeping back as well.
Please keep in mind that the weather can be unpredictable as the Himalaya creates its own climate, regardless of the season.