You could apply now since your within the 45 days early window that Chaeng Wattana allows.
I would say the best days are Tuesday, Wednesday and Thursday.
Remember there are holidays on the 6th and the 13th to the 15th of this month and the 2nd, 4th and 13th of May.
You can make a appointment to do your extension here.
https://outlook.office365.com/owa/calendar/ImmigrationOfficeGovernmentComplexChaengWattanaRd@imm1division.onmicrosoft.com/bookings/