Over the years I have noticed that most alt season started as follows:
1. $BTC pumps first.
2. $ETH $BET & $LTC coin pumps next, leading indicators of the Alt season.
3. Big marketcap tokens next. Usually top 20 on CMC.
4. Then full blown Alt season.
Just because it happened so in 2017, there is no guarantee that it will repeat in 2021. Back in 2017, there was a lot of hype surrounding the ICOs after a few successful ones gave 10x or 20x returns to those who invested in them. But this time, the altcoins are moving up, just because of the spillover effect from the Bitcoin rally. There is hardly any improvement in the ground situation for the alts.