comi al almuerzo
is one way of saying "ate lunch" "comer al almuerzo"?
It depends where you live. For me and all the people that I know "almuerzo" is brunch. Breakfast is desayuno, brunch is almuerzo, lunch is hora de la comida, snack is merienda, and dinner is cena. Sometimes people ask you Que comiste a la hora de la comida? What did you eat for lunch? I can tell them if I ate a "brunch" style food, like eggs, bacon, pancakes, like a heavy style breakfast, or burritos, tortillas with machacado/huevo, chorizo, etc. I can tell them that I actually didn't have regular lunch, that it was a brunch at lunch time. Usually if you have an almuerzo, probably you won't eat lunch. That's my experience.
In Colombia, almuerzo is definitely lunch. Almorzar is to eat lunch. I'm not disagreeing with you Sylyon, just clarifying. Where are you?
You can say...Comí el almuero a las dos. I ate lunch at two o'clock.
Or you can say...Almorcé a las dos. I had lunch (ate lunch) at 2:00.
You just say "almuerzo" that already means I have lunch. "Verb almorzar"
Past tense: just use the preterite or imperfect form of "almorzar"