The 2021 year-end tax planning guide emphasizes the importance of reviewing tax and financial affairs before the April 5 deadline, with strategic advice tailored to individual circumstances, especially considering the pandemic's impact. Key points include understanding personal allowances, effective use of pension contributions, and tax-efficient ways to manage income among family members. Additionally, it highlights tax-efficient investments and the implications of capital gains, as well as considerations for charitable donations and familial company structures.