A workout day is a routine you have given a name to — Push day, Legs, Monday. It is a list of moves that belong together, not a date in the calendar.
Making a routine
Open Workout days and add one. Give it a name and save — you have to save the day before you can add moves to it.
Then pick the moves that belong to it. You can drag them into the order you actually do them in, and remove one at any time. Removing a move from a routine does not delete the move itself.
What a routine gives you
A shorter move list
In the move selector you can filter by a workout day, and only that routine's moves are shown. Salivihko remembers the choice, so the next time you open the selector you are still in the same routine.
The whole day in one press
On the Workout page you can add every move of a routine at once, ready and empty, and then just fill in the numbers as you go.
When a routine falls out of use, hide it instead of deleting it.
Letting the app choose
If you do not know what to do next, open a category's menu in the move selector and pick "Randomize". Tell it how many moves you want — three by default — and it suggests some.
The first suggestion is not random at all: it offers the moves you have gone longest without doing, which is usually exactly what you have been avoiding. Press again and the suggestions become genuinely random.
Moves you have already logged that day are left out, so you cannot be handed the same thing twice. Press "Add moves" and they appear on the day, empty and waiting.