Although "conciliar" means to reach an agreement, it's not really used for arranging a meeting or an event. It's rather used when there is need to make compatible things that seemed contradictory, or to bring an agreement where there has been dispute.
It was probably used if all the people involved had an objection about the activity. Yet, to make sense from it, do you remember the precise sentence where it was used?
As for your proposal "Vamos a hacerlo todo junto
en el mismo dia": it's long but neither low class or incorrect.
I'd prefer "vamos a hacerlo
todo el mismo día", if you are talking about
many things that have to be done the same day. "Todo junto", as you wrote it, adds emphasis to the fact that all activities will be done at the same time.
If it's a
group of people who will do the same things on the same date and will
gather in one place to do it, then I'd say "vamos a hacerlo
todos juntos el mismo día".
"En" is not incorrect, but I find it redundant.
Side note: "conciliar el sueño" is definitely the most common expression using that verb.
However, I think you've found things like:
- ¿Cómo haces para conciliar la vida laboral con la familiar?
- Es difícil conciliar la austeridad con la eficiencia del gobierno.
- No se puede conciliar la explotación de recursos con la conservación de los ecosistemas.
- El embajador realiza una labor conciliadora entre los dos países en conflicto.
- El director hizo un papel conciliador para resolver la crisis entre la empresa y los empleados.
- Al estallar la crisis, los países vecinos le pidieron al presidente una actitud conciliadora con los rebeldes para evitar la guerra civil.