What date do i make the rent increase notice on a place where i am inheriting tenants

[Irwin White-Gittens]

I am in the process of buying a property with 3 tenants. One tenants pays the 15th of the month. I am trying to increase the rent. Sending out the rent increase notice. in contract from the prior seller, there is a 30 days notice needed. Do i give them the date till mid july or make it the first day of the month which would be august. Please let me know. Thank you