The advice from the Belgian authorities is clear: plan your holidays around your vaccination appointments!
The following link takes you to a Flemish government website that will show you the target week for you to get your invite for a vaccination appointment by post. The second date shows the target week for your first vaccination shot. Depending on the type of vaccine you are getting, detailed in your invite, you can calculate the date of your second shot, if one is necessary. The tool is intended for people without underlying health issues. Fill in your age in the box where it says 'vul hier je leeftijd in' and you will see the target week for you to receive your invite followed by the target week for your first vaccination. Your second appointment depends on the vaccine brand, but you will only know your brand when you get your invite by letter.
In theory you can change the date of your first appointment by up to 30 days, but because so few free slots will be available finding an alternative that suits you may be difficult.
The timing of the second shot is a lot stricter because the vaccine schedule needs to be observed. Let the vaccination centre know if you can’t make your second appointment to avoid waste, but you won’t be able to make an alternative appointment at will. After the summer, when the vaccination campaign is complete, the government will examine how people who missed their second shot or even both shots can get vaccinated.