As most of us know, we celebrate two days of yom tov in chutz l’aretz (CL”A). The famous question is asked, that if nowadays we rely on Hillel’s calendar, what is the purpose of having two day’s of yom tov? We all know exactly when things are going to fall out, so we should have only one day of yom tov. This would be great for a bunch of reasons, including no 3-day yom tov’s, extra day of expensive food, and not overeating.
The classical answer is given that since in the olden days, when we weren’t sure exactly when Rosh Chodesh was, thus we were unsure of the exact date of yom tov, even though we now know when things fall out, we’re just doing it k’minhag avoseinu.
I was listening to the OU daf yomi podcast from yesterdays daf (Beitza 4b) and the maggid shiur brought what I thought was a great answer from the Meshach Chuchma. He said then in the days of Mashiach, things are going to return to the old ways, including rosh chodesh and two-day yommim tovim. If were were to rely solely on Hillel’s calendar, it would be showing a lack of emunah regarding the coming of Mashiach. By continuing to have the two-day yomim tovim, we’re showing our emunah that Mashiach will come soon by continuing the schedule as we have it today.