Get out! I'm renovating. When to break the news to tenants?

[Kevin Scanlon]

I purchased an occupied duplex earlier this year in Mystic, Connecticut. It's a Greek Revival style building originally constructed in 1845. The location is very good (although it's in a historic district).

The house is habitable, but is in need of major updating and structural repair to bring it back to it's former glory.

The property will generate close to 3 times the rental income once it is renovated so it will be well worth the investment. 

I would like to begin the work in September, 2015. The project will require the whole property to be vacant and will last 3 to 4 months.

Tenants in unit 1 are on a month-to-month lease. Tenants in unit 2 are on a lease that ends in April.

Ideally, I want to maintain occupancy in both units until September, but also want to mitigate the risk of having to formally evict anyone that refuses to move which could delay the start date of the renovation.

When should I inform the current tenants of my plan? What strategy would you use to ensure they stay until September, but move out on time?

Thanks in advance for your advice!