The best months to trek to Annapurna base camp are pre-monsoon are February, March, April and May. If you are set on trekking post monsoon, I high recommend trekking in late September, October, November and December, furthermore early September is also an option but I prefer the months listed above.
Please plan for months of April to June mid. These are typically pleasant moments which you can enjoy the trek and also soak in the Himalayan range in its full glory Sept Oct Nov you can do the trek but it might be less crowded So plan and go. It's a difficult trek and you need to be physically fit to get it dome.