If you complete your Bachelor programme in June 2021 and continuing on a Master's programme in September 2021, you are eligible for receiving SU over the summer holidays.
You have to apply for SU for your Master's programme from 1 July 2021. Please note, however, that you are only able to apply for SU one month before payment. This means that the earliest you can apply for SU is 1 June when applying for SU from July. The deadline for applying for SU in July is 31 July 2021.
If you want to apply for SU you have to fill out an application form at su.dk via the online system minSU ("my SU").