I don't think you are in Ottawa, but this is what Ottawa Public Health put out in their COVID vaccine update newsletter today:
First dose before April 18, 2021
Individuals who received their first dose of Pfizer or Moderna on or before April 18, 2021 will also be eligible to schedule an appointment through the provincial booking system to receive their second dose beginning on Monday at 8 am.
I got mine just after that cutoff so it will be a little while longer until I can use the system to book a future date for dose 2.
I don't know if your local public health unit has an email list for covid vaccine news updates, but I signed up for Ottawa's a couple weeks ago and have found it quite helpful and informative; I recommend seeing if your PHU has something similar and if they do signing up for it.
This is the Ontario press statement about the updated to the AZ second dose guidance (to allow mixing of vaccine types); it has some useful info that I think will give you some answers:
https://news.ontario.ca/en/release/1000256/ontario-updating-astrazeneca-second-dose-guidance including:
QUOTE
Effective June 4, 2021 and in alignment with NACIβs recommendation, Ontarians who have received their first dose of the AstraZeneca COVID-19 vaccine can choose to either receive a second dose of the AstraZeneca vaccine, or an mRNA (Pfizer or Moderna) vaccine for their second dose. Both second-dose options available to individuals who received their first dose of the AstraZeneca vaccine are safe and provide strong protection against COVID-19 and will count as a completed series. These second dose options will be provided at the recommended 12-week interval.
Beginning June 4, 2021, individuals who received their first dose of the AstraZeneca vaccine 12 weeks ago and who would like their second dose of the AstraZeneca vaccine, can contact the pharmacy or primary care provider where they received their first dose to book an appointment. Individuals who received their first dose of the AstraZeneca vaccine and are opting to receive an mRNA vaccine have the option to schedule their second dose appointment at a
participating pharmacy where the Pfizer or Moderna vaccines are administered. Primary care settings and pharmacies may also be reaching out to eligible Ontarians.
In addition, expected the week of June 7, 2021, individuals who received their first dose of the AstraZeneca vaccine and who choose to receive an mRNA vaccine for their second dose can register for a βsecond dose onlyβ at a 12-week interval through the
provincial booking system. Eligible individuals will also be able to schedule their second dose appointment directly through public health units that use their own booking system.
END QUOTE
Hope this helps
SW