For a 6-team league, you can simply run 10 games every night, using the 6-team tournament grid; perhaps using slightly shorter games. But you might want to run 6 games every night, in which case a suitable grid is shown below. It takes 5 weeks to play.

15 games are played by each team.
30 games are played in the whole tournament.
Each team plays every other team 3 times.

Game
A1B1C1D1E1F1G1H1I1J1K1L1M1N1O1P1Q1R1S1T1U1V1W1X1Y1Z1A2B2C2D2
Team1R Y G Y R G G R Y G Y R Y R G
2GR Y GY R Y G R RG Y R YG
3 GR Y GY R R Y G RG Y G YR
4Y GR GY R GR Y Y RG G R Y
5 Y GR R GY Y GR Y RG R GY
6 Y GR R GY R Y G Y RG Y G R

6 team tournament grid
5 team League Grids 7 team