Salam `Aleykum, I don't believe there is a specific time for this??? but if you want to wish someone a blessed month then you can do this at the beginning of the month or maybe 1 day before Ramadan. For example let's say that Ramadan is Sunday, and I work in a company 6 days a week and we have Sunday off, so in other words I won't be able to see my co-workers on the first day so I'd wish them a blessed month on Saturday... 1 day before Ramadan. it's up to you.