According to a tweet from Nintendo’s official Splatoon Twitter page, the game will be receiving a free one-week’ demo’ of the game starting on March 26th until April 1st.
The demo will allow you to play Turf War, Ranked Battles, and Salmon Run. You will be able to play friends who own the game in this demo as well. This special one week demo will not require a Nintendo Switch Online account to participate. Saved progress will as well transfer over if you purchase the full game.