I cant understand why this phrase translated to "What time does Dad get up on Mondays?"
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The simple translation into English would be: "At what time does dad awake on Mondays?".
hora: time (hour)
se levanta: awake[s] [himself], get[s] up [himself], lift[s] [himself], get[s] [himself] [off the bed], rise[s] [himself]... The verb "levantarse" means to lift or to rise, and it is often said in this context implying that he will rise his body up off the bed, to become vertical, and to stop lying horizontally on the mentioned bed, because he was asleep
los lunes: [the] Mondays