Tomorrow is Vallabhacharya's appearance day as it happens on the 11th called Varuthini Ekadashi. So if you are doing a fast, by all means do it, but most importantly do an Upvaas by either visiting one of his 84 Baithaks or studying his books and feeling close to him. It should be interesting to understand how this Upvaas (उपवास) phenomenon came into existence and then decide what is its relevance today.
In ancient times, the daily activities people used to do were very limited and time consuming - a bit of farming or hunting, cooking the meal, a bit of socializing and repeat the cycle before it gets dark. Since there was no refrigeration, they depended heavily on grains as "stored" food and since there wasn't electric motor controlled flour mills, they needed to manually stone-grind the grains to make flour or burn lot of firewood to boil them in open air. This cooking was real time consuming process of the largely illiterate folks depriving them any opportunity to listen to the discourses from Brahmin (ब्राह्मण = a learned person) and acquire higher knowledge about life.
So Brahmins came up with a formula called Upvaas. Once in a fortnight, you should not spend time on cooking the grains and instead eat fresh fruits and vegetables which needed no or very little cooking. And also eat only once on that day. This process will save you some time that you can use to "Up+vass" (nearby sit) to a learned man and listen attentively to his teachings about higher aspects of life that will help you live a better, more fulfilling life. The side benefits would be learning of self-control and detachment, a good stamina building exercise for hard times and cleansing the stomach of any irregularity thus improving your health. The folks were sold on this and thus started the Upvaas tradition on 11th of every fortnight. The Brahmins would also receive gifts (food grains etc) in return from the people, so it was a win-win situation.
Fast forward to today. 1) People are not illiterate (at least the ones reading this blog) 2) The cooking activity is not that much time consuming - grain or no grain and 3) People no longer need to "sit nearby" physically to a learned person to receive his/her teachings thanks to the scientific advancements. That is to say, you can continue to do physical Doorvaas (दूर:वास) and still acquire knowledge by mental Upvaas or instrument-facilitated Upvaas (उपकरण-साधीत उपवास). (e.g. you can do Upvaas of Mumbai-based Shyamubava by listening to his online pravachan or by making a phone call or a video chat). In fact we can choose whom to listen and whom not, as well as verify their teachings with respect to the original and modern scriptures that are available in text form (digital or physical) compared to the verbal only form of ancient times.
Where does that leave us with regard to Upvaas? The answer will come from its definition explained above. Upvaas is not a discrete phenomenon as set at the beginning, it is a continuous phenomenon. Right from the time we are born to our last breath, we stay somewhere or the other i.e. we do Vaas. It's up to us to make that an Upvaas (staying at a good place) or Apvaas (staying at a bad place). Upvaas can be performed any time for any period e.g. you can say "today evening I'm gonna do Upvaas of Shree Vallabhacharya by studying Anu-Bhashya" or "Wow, we did Upvaas of Shrinathji for 3 days at Nathdwara." Better than Upvaas is Antahvaas (अन्तःवास or अन्तेवास) meaning staying inside a divine environment. If you perform Gruhseva (house worship of a deity), you are covered for Upvaas all the time, it could even become Antahvaas if your consciousness is such that it is the Lord's house in which you are living as a servant.
Upvaas has very little to do with food - just eat Sattvik food and you are good to go. The word Falahar (फलाहार) was used to suggest eating only fruits, but today it means all kinds of fancy food preparations that "conform" to the food-standards of doing an "upvaas" e.g. flour of some grains, salt of certain kind and many other on-the-edge items. Lest you eat outside the standard, your Upvaas is gone! Give me a break, people. It is also an old joke now that many a people would do Upvaas to have a delicious sumptuous meal.
So, in conclusion, Upvaas is not a synonym for fast. A fast can have many benefits, but the main one was supposedly to be able to do Upvaas. On the other hand, Upvaas doesn't need a fixed date, it could be done any time, however brief or long. Main goal of Upvaas is to uplift your consciousness with higher knowledge and understanding, primarily about the Supreme. Upvaas is nothing but an education process.