Most of the airdrop is unannounced and retroactive to reward user that is genuinely supporting a certain project in the form of various task such as staking, running a node, trade and etc. It’s very hard to determine exact airdrop date but some website like https://boxmining.com/upcoming-crypto-airdrops/ gives you a list of project that has the potential to have an airdrop but again some of them is not guaranteed.

You will need to risk completing the task without any guarantee for an airdrop. It requires some spending and this is now the format of airdrops.