You just fixed the property issue last month and you tell the tenant to move out the following month. I can tell you three major arguments the tenant would bring to judge.
1. Retaliation - For sure the tanant will bring this reason to against you in court. In your state maybe a bit different, but in California, the judge will sure beleive it becuse of such short time and no reasonable cool down periord. The judge doesn't look at the day you ask the tenant to be out. The judge looks at the day you noticed the tanant. The judge knows your decision was made way before you noticed the tanant which is very possible your decision was made while you were fixing the property. If I'm the judge or jery, I would consider it.
2. The tenant will suspect you try to kick her out and re-rent to other people with higher rate to gain benefit. Because you spent money to fix/improve the property and now you are trying to recover the expense. You must spent good amount of money to fix the property since it took 4 months. (not sure if this is your real reason)
3. The lease contract- Because you sent her your signed copy, that means you already agreed to let her stay until August. She didn't sign the lease because she wanted to be able to break out of contract anytime. Once you signed the contract, you agreed and binded to the contract. It doesn't matter the other party agreed/signed or not. The bottom line is - you agreed. LemonCoconutJuice is right, the tenant can sign the contract any time. She can even sign the contract in front of the judge to end the case and kick you out of the court room. So this is the dead end for you.
Personally I would suggest you wait until June and tell the tenant that her lease is ending in August and you don't want to continue to lease. This would make you look much better in front of the judge for the following reason.
1. You have reasonable cool down periord to prove you are not retaliation. By that time maybe the tenant forget and doesn't feel she is retaliated and simply move out without fighting. Maybe you cool down enough and change mind and let her stay longer.
2. You are not in a hurry because you are giving her two months advance notice.
3. You are not tripped by your own contract.
Other advise -
Check or modify your future lease contract to make sure you have a paragraph to define how and when the contract should be enforced. An example is " This agreement shall be enforced/valid only after both parties signed....."
Never sign the contract before the tenant signs it. Ask them to sign first, mail back to you, you sign, and send the copy back to tenant.
Have another paragaph to define what happen when any party need to terminate the contract. if you don't have it and you send a lease termination notice to the tanant, the tenant may claim there is no more contract between you and them, and release from the contract restriction such as pay day, no pet, no unlawful use...
Keep a chinese sentence in mind - "try to get faster would never get there". This is very true in many cases.