If you got your ILR yesterday, you have to wait 365 days to apply for British citizenship as you must have completed 1 year of ILR before applying for naturzlisation and you must have also been in the country for 5 years. For example some other visas (student, work permits etc) allow you to process further application 28 days advance. But for nautralisation, the earliest you can apply is on the 1-year-1-day th as far as I know.