En Navidad

Hola Inez

In our sentence: "Abrimos muchos regalos en Navidad" - We opened lots of presents at Christmas, you could have also used "por Navidad" but it would carry a different nuance, that of a "why".

"En..." is more literal, "en" pinpoints a time frame , like saying “on Christmas Day” or “at Christmas time.”,  it can refer to the day itself or the period around it.

"Por" implies a reason, cause, or occasion  -  like “for Christmas” or “because of Christmas.

I hope this clarified it.

Saludos cordiales

Inma