With version 5.7.25 of Google Calendar published on June 15, the app added one feature that we've always wanted and sort of gave up on having: drag-and-drop for events. But it took until Reddit user Racing24 noticed it yesterday for us to go back a few versions and track the change down to that specific release of Calendar.
How it works is simple. With your Calendar open on the day view, 3-day view, or week view, you can hold any event and drag it to another time slot. In the day view, this simply means changing the time it's scheduled at, but in the 3-day and week view this also allows you to move an event to another day (or even week or month if you hold the event to the edge of the screen long enough).
Moving an event around in day view (left), 3-day view (middle), and week view (right).
While you're moving it around, the previous scheduled time slot becomes more transparent and the event hovers on top of your calendar until you drop it. You can choose between 15-minute increments around each hour, but any more granular control requires you manually edit the event. Oh and if you do something by mistake, an Undo button will show up briefly to let you quickly get things back in order.
If you're running any version of Google Calendar post-June 15 and v5.7.25, this drag-and-drop gesture should be working for you. Though I have to note that I did notice it wasn't always functional for me in the 3-day view, so your mileage may vary on its responsiveness.