When will my IRA retirement account payer advise me of my RMD?

I will be 70 in Sept. 2019. When will my retirement account payer tell me my RMD? Since I will be 70.5 in 2020, I may take my first RMD in 2020, rather than take 2 in 2021.

If I decide to take 2 distributions in 2021, why does the IRS require the first RMD be taken by April 14, 2021 and the second by Dec. 31, 2021? I know that the distributions are based on IRA values at the end of 2 different years. I would have thought both RMD's would have to be taken by Dec. 31, 2021.