This not really a practical question, as I’ve no intentions of taking advantage of it, but I want to make sure I know the answer.
According to canon law, abstinence is not required when a solemnity falls on a Friday. Thus, on the Solemnity of St. Joseph last year - a Lenten Friday - meat was permitted. Similarly, meat will be permitted next Friday, which will be the Solemnity of the Annunciation.
Now Solemnities, unlike Feasts or other lesser celebrations, Liturgically begin with the first vespers on the night preceding the date of observance. Thus, the Solemnity of St. Joseph begins with first vespers tonight, and the Solemnity of the Annunciation begins next Thursday night.
As far as I understand it then, the law of abstinence should not apply following first vespers tonight. However, I want to make sure that I understand this correctly. I know that in canon law a day is considered a period of 24 hours beginning at midnight, so it’s clear that at midnight tonight the Solemnity would be “in full force.” However, I have not seen any official document indicating when a Solemnity is officially regarded as beginning. There are some important questions surrounding it, too. For example, how is the solemnity reckoned for those who do not take part in a Liturgical celebration of Vespers or Mass?