PT-PT

Ontem

Ontem, a minha família e eu enterramos a minha avó. O dia era foi agridoce, porque também celebramos o aniversário de da minha sobrinha. 

The mistakes are in red and suggestions on how to improve the sentences are in blue, based on how we speak in Portugal.
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Ontem

Ontem, minha família e eu enterramos minha avó. O dia foi agridoce porque também celebramos o aniversário de minha sobrinha.

Meus sentimentos!

Ontem

Ontem, minha família e eu enterramos minha avó. O dia era foi agridoce porque também celebramos o aniversário de minha sobrinha.

---Notes---

1. Since you're referring to a specified, recent day, you need the Perfect Past of "ser" - "foi".

2. The term bittersweet has no normal sounding translation, at least in Portugal. My Brazilian equivalents would have to state their view on this. You CAN use "agridoce", but this word is mostly used to refer to the sweet and sour sauce, so its use here, albeit correct, sounds somewhat quirky.

3. In Portugal we use definite articles before the possessive, but I don't think they do it as much in Brazil, so I left that alone.