If the water is 12C it is ideal as it is before algae starts growing – any later and the work and cost rapidly increase.
An early start also gives time to fix problems but if the pool was properly prepared for winter the water should be blue.
A deep clean is essential, even if the water looks fine.
Clean the edges and border of the pool, skim coarse impurities, scrub sides and bottom, clean and scrub water line then vacuum the bottom before setting up the robot cleaner. Check filters do not get clogged.
Renew at least one-third of the water to reduce levels of product and cleaner residues.