Can you use Chord of Calling during your opponent's end step to put a creature directly onto the battlefield?

Chord of Calling is an instant (CR 101.4 confirms instants can be cast any time you could cast an instant), which means you can cast it during your opponent's end step as long as you have priority. It searches your library for a creature card with converted mana cost (mana value) X or less and puts it directly onto the battlefield — no summoning sickness 'delay' from casting; the creature simply enters the battlefield.

Convoke (CR 702.101) allows you to tap creatures you control as you cast Chord of Calling to help pay its mana cost. Each creature you tap pays for one generic mana or one mana of that creature's color. This can make casting it during your opponent's end step very efficient since your creatures are likely untapped after attacking and blocking earlier in the turn.

The creature entering the battlefield during your opponent's end step is still subject to summoning sickness (CR 302.6). However, since summoning sickness only matters for attacking or using tap-symbol abilities, and it resets at the start of YOUR next turn, the creature will be fully available for your next turn — which is a key reason players deliberately cast Chord of Calling at end of turn.

Concrete example: It is your opponent's end step. You cast Chord of Calling with X=4, tapping two of your green creatures via Convoke to help pay the cost. You search your library and put Siege Rhino (mana value 4) directly onto the battlefield. Siege Rhino's enters-the-battlefield trigger fires immediately. On your next turn, Siege Rhino is ready to attack.

Relevant rules: CR 101.4 (casting instants), CR 307.1 (instants), CR 702.101 (Convoke), CR 302.6 (summoning sickness), CR 116.1 (when you may cast instants).