Buying reward cards is the cheapest way to increase your collection and you will also get more sps airdrop competed to other more expensive cards.
Though there's no any exact best time to buy cards but if you want to buy cards for cheap then l have some tips for you which can be helpful.
For example best time to buy reward cards is after season reward card distribution because after reward distribution supply of the cards increase significantly and most of the people sell cards for cheap price which is best time to buy reward cards.