The deadline for making contributions to a solo 401(k) is not year-end. The deadline for making your election to make an employee contribution is year-end, but the deadline for actually depositing your employee contribution to the solo 401(k) is as soon as is practical after determining your net earnings, but not later than the due date of your tax return. (You should still try to determine your net earnings as early as is practical.) The deadline for making the employer contribution to the solo 401(k) is the due date of your tax return. You'll simply tell E*TRADE that both are for 2020.
