When Magic: The Gathering Arena ushers in a new set, generally the powers that be provide a promo code that you can redeem for three packs of said set. The new code is actually already live now (PlayM21), but a few are cycling out as of today (the end of June 30).
For reference, those three codes are “PlayRavnica,” “Play Allegiance” and “PlayWarSpark,” which will net you packs of Guilds of Ravnica, Ravnica Allegiance and War of the Spark respectively. “PlayM20,” “PlayEldraine,” “PlayTheros” and “PlayIkoria” are also still available to claim, and will be for some time.
As a reminder, all three of those sets will rotate out of standard (the main form of competitive play) in the fourth quarter, in addition to Core Set 2020. Instead, Throne of Eldraine, Theros: Beyond Death, Ikoria: Lair of Behemoths, Core Set 2021 and Zendikar Rising (due in September) will remain active going forward.
Source: Destructoid PSA: These Magic: Arena free pack codes are expiring later today