All Summon Gates — with the exception of the Daily Summon and the Epic Troop Summon — offer with each Summon a small chance of drawing the current Hero of the Month as a free bonus. These Heroes can be obtained this way only during the calendar month (based on UTC+0) of their release.
One new Hero of the Month is introduced to the game every month. The new one replaces the old one at 07:00:00 UTC on the first day of each month. Please be aware that the switch does not happen at midnight UTC!
Past Heroes of the Month eventually become obtainable again once they join the Tavern of Legends 12 months after their release. Legends Coins can be earned during this event and used to make free Summons at the Legends Summon Gate.
Please note that the Legends Summon Gate can also summon Classic Heroes. You can always check the exact probabilities by tapping the blue circle icon with an "(i)" in the top left corner.