It will start either on Friday or Saturday.
By sunset today, if it is not announced that tomorrow is Ramadan, then it will surely be on Saturday.

"Mubarak" means holy as a noun. As an adjective, it means greetings.

"Kareem" means kind and noble.

As for the use of both, it goes like this:

We say "Ramadan Mubarak" to greet each other with the arrival of the holy month.

We say "Ramadan Kareem" to someone or to a guest when we meet them and cannot offer a meal or a drink because we are fasting. It is a symbol of offering hospitality, but yet practically unable to do it out of respect for Ramadan.