On 25th April 2019, Famitsu revealed a lot of new information about Three Houses. Please refer to this article for all the details. Also, we wrote a follow up article with additional information here.
Afterwards, the official Japanese Fire Emblem Twitter started posting character profiles on a semi-regular basis. Later, on 10th May 2019, the Japanese official website was opened, which included all the profiles revealed up to that date.
Above is the official Heroes event calendar for 10th May to 11th June 2019.
Highlights include a (presumably) Bridal-themed Special Heroes banner on 21st May, a new Mythic Hero at the end of May and finally the next New Heroes will be coming on 11th June.
Please check this article for a text breakdown.