For over 20 years we (Luis & Jo) have had a bird box in our garden which has had a nesting pair of Blue Tits every year. A few years a go we added a camera to see the birds nesting in action.
Blue Tits lay 1 egg a day over several days usually towards the end of April and incubate their eggs for about 13 to 15 days. Once the eggs hatch the female bird keeps the chicks warm while the male bird supports her and bring her food.
The first egg was laid 18/04 and mummy bird starting sitting on 26th May. 13 days later on 8th May the eggs began to hatch.