Use Nominative to say what the date is.
— Какћй сегћдня дљнь? "What day is it?"
— Какће сегћдня числћ? "What date is it?"
— Сегћдня средђ, вторће мђя. "Today is Wednesday, the 2nd of May."
— Зђвтра бџдет трљтье. "Tomorrow is the 3rd."
— Вчерђ бІло пљрвое. "Yesterday was the 1st."
— Какћй сейчђс мљсяц? "What month is it?"
— Сейчђс мђй. "It's May."
Use Genitive to say 'on' a calendar date.
If you mention the name of the day of the week, recall that on a day is в + Accusative.
If you include the month, use Genitive, just as you use the English word 'of' in this context.
— Когдђ бџдет контрћльная? "When will the test be?"
— Контрћльная бџдет в срљду, вторћго мђя. "The test will be on Wednesday, the 2nd of May."