There are no hard and fast rules to what makes up Altcoin season. Altcoin season is generally referred to as a period where Bitcoin dominance is falling and while the market capitalization of alternative coins continue to increasing.

In the beginning, there was only Bitcoin. Bitcoin held 100% of the cryptocurrency market. As other cryptocurrencies began to enter the marketplace, these 'other' cryptocurrencies were called 'alternative coins', more commonly referred to as "altcoins".

Altcoin Season is typically referred to as when Bitcoin is dropping in market capitation while Altcoin increasing by a considerable rate (when compared to Bitcoins drop).

As the market continues to grow, and we have currently more than 10,000 different cryptocurrencies being actively traded, the dominance of Bitcoin will likely continue to fall.  It could be said with the continuing increase in altcoin popularity and continuing decline in bitcoin market dominance, it's always altcoin season.