Sweet peppers are best planted after the threat of frost has passed and weather has warmed. I usually wait until the last week of May here in Minnesota Zone 4.
When I plant my peppers, whether sweet or hot, I plant them roughly 15 inches apart. Now that may seem close for how large a pepper plant can get, however, I’ve found that planting them too far apart can leave some peppers exposed and causes sun scald on the fruit. When planted closer together, you don’t sacrifice production and the plants work together to protect the fruit.
Once planted, I often place a good 5-6″ layer of mulch around the row. If I’m running soaker hoses, I do so before laying the mulch. My mulch often consists of grass clipping or straw. Both will compost down throughout the season; improving your soil and feeding the plants.