Answer:
Jueves 31 de diciembre.
Step-by-step explanation:
Si hoy es martes 29 de diciembre, el jueves será 31 de diciembre. If today it's December 29th, Thursday will be 31st. In Spanish months and days of the week aren't written in capital letters. Apart from that, the number of the day goes before the month, as opposed to English: 29 de diciembre / December 29th.