The card Tablet of Time's Dawn (ToTD) states:
You may banish this Construct to take an additional turn after this one.
Is there a best1 time to use the card's ability? I see three possibilities (my definition of the three phases are somewhat vague):
- Early game (basically as soon as possible): if you're trying to thin your deck (e.g. by using Shade of the Black Watch, which lets you banish a card in your hand or discard pile), playing ToTD now might get you a more effective (more runes or power per card) deck than your opponent.
- Mid-game (you'll get several opportunities to use cards bought in this phase): playing ToTD now, will give you two shots at buying good cards in the card row, thus denying your opponent them.
- Late-game (your last, or second-to-last round): playing ToTD late in the game will give you two chances to use your deck to the best of its ability.
This is of course barring the occasion that you get e.g. Hedron Link Device and Reclamax in addition to ToTD, for the possibility of infinite turns. Let's assume the game is played with 60 VP and you're playing against one opponent.
1. Where best is defined as what gives you the greatest likelyhood of winning the game.