The Democratic Party awards bonus delegates as a reward to locations that wait until later in the calendar to hold their primary or caucus. With the 2020 election calendar mostly set, a pretty good estimate of those bonus delegates can be calculated. 

The following remains subject to change should any calendar dates change. 

24 locations (21 states, DC, Guam and Virgin Islands) will receive a total of 211 bonus delegates for holding their 2020 nominating contest after March 31.  These are pledged delegates, bringing that total to 3,979. As such, the 'magic number' for winning on the first ballot is increased from 1,885 to 1,990. The total number of delegates, including 766 automatic delegates^, is now 4,745.

From an overall calendar perspective, this slightly reduces the proportion of pledged delegates available before April.  The pre-bonus estimated distribution:

There are two types of bonus delegates: Calendar and Cluster. The calendar bonus is 10% for locations holding their contest in April; 20% for those in May and June. The cluster bonus is an additional 15%, and is basically for holding a nominating contest on the same date as adjacent locations. In 2020, the only cluster bonus will be for six east coast states that have scheduled their primaries on April 28.

April (10% calendar bonus):  Alaska, Hawaii, Louisiana, Wisconsin, Wyoming

April (25% calendar + cluster bonus):  Connecticut, Delaware, Maryland, New York*, Pennsylvania, Rhode Island

May/June (20% calendar bonus): District of Columbia, Guam, Indiana, Kansas, Kentucky, Montana, Nebraska, Oregon, New Jersey, New Mexico, South Dakota, Virgin Islands, West Virginia

Note that the bonus is calculated off base delegates, which are approximately 85% of pledged delegates.

^ The Party's official term.  More commonly known as superdelegates.  With the addition of the bonus delegates, the proportion of automatic to total delegates is slightly reduced.

* Although likely to be April 28, New York's date remains tentative. As the largest recipient of bonus delegates (49), a change here would have a noticeable impact on the total bonus delegates.